Thanks. I'll add more light and see if that perks it up. It's the first time I'm using the new rack so I wasn't sure the light would be sufficient. The mature lettuce I'd already had growing for a few months is doing great once I moved it into the rack, but I wasn't sure how new seedlings would do.

The other container of spinach seedlings that's doing better is more centered under the light, so that may very well be the problem.

You might describe your lighting system. How far away from the plants are the light bulbs? Most lighting systems require that the bulbs be just inches over the foliage. With LEDs and fluorescents, you can do that without frying the plants. Farther away, and the intensity is just insufficient. As it is, even the best lighting systems are no match for full sunlight. "Reading light" illumination is nowhere near enough for plants.

I started off with 2-tube LED shop lights for each shelf (24", the width of each shelf). I had them positioned about 6" above each pot, but I just lowered them. I have the entire enclosure surrounded on all 4 sides (and the top) with Reflectix to bounce the light around to get more out of it. That seems to work well for mature lettuce, but I wondered if it would be enough for seed starts. I'm not really seeing anything get leggy, so I thought it might be enough. But now a few things are drooping as the true leaves start coming in.

I have more lights I can supplement with. I'll give that a try.

That looks like a nice setup. Maybe it wasn't lack of light that caused the droops. Of course, lack of moisture is another. I'm a little skeptical about moisture meters. Did the mix feel truly moist at depth?

It didn't stick a finger down in it (pots aren't really big enough without disturbing the seedlings) but I'm pretty sure the soil is reasonably moist (if anything, maybe too wet). That's usually the first thing I do (water more) when something doesn't look good. Usually I just give it sips of water daily at this point until it gets bigger but when it started drooping a few days ago I made sure to give it a thorough watering until it started dripping out the bottom.

I'm starting to worry that I have something wrong with the soil, because the tomato plant seedlings that just broke the surface 2-3 days ago are starting to look sickly too (kinda shriveling up). If they don't look better tomorrow, I'll try a test by pulling a few of the packs out of the grow rack and see what they do on a window sill instead to compare.

How wide is the plant stand? If 2 feet, light is minimal. If 4 feet, light is greatly improved.

And, as has been said, lights should be just above the tops of the seedlings/plants.

Further, light is more intense under the center of the tube rather than at the ends of the tubes.

Then, too, how many hours are the lights on?And Wat are the temperatures -- during the day and at night?

I use some light duty chain and S-hooks to adjust my grow light up and down.

They're 24" tubes, but so is the plant rack. It's the similar amount of light output per inch as 48" tubes are, meaning the light directly above each plant is the same regardless of tube length. But a longer tube provides more angular light coming in from the side to each plant. I'm not sure how helpful that angular lighting is to a plant. Probably not much for seedlings (thus why we lower the fixture to just a few inches above them). But I suspect when larger leaves are grown, that angular light provides more benefit.

That's why I enclosed all 4 sides with Reflectix, to give it some boost on light from the sides. I know reflection isn't as strong as the light source itself, but every little bit helps. And enclosing all 4 sides really does seem to help significantly. It's pretty bright when peeking in from a corner when the front cover is rolled down.

I mounted the lights in a way that takes just a few seconds to raise/lower them. I also have some free-hanging sockets that are easy to hang if I need a temporary boost. I added a 1600 lumen bulb above the spinach pots yesterday and I'll see if it makes a difference over the next few days. But since seed starts don't appear to be getting leggy, I think the problem may not be lighting.

I have the lights on 16 hours/day. Temps inside the enclosure range from a low around 58-60 at night to 72-77 during the day. It's in the furnace room which has a vent pipe to the outside, thus gets a little cooler at night.

What soil are you using? What I see in your original picture looks like a lot of uncomposted woody debris. A very loose soil structure like that won't offer seedlings much support, and probably doesn't hold moisture that well. Have you used that mix successfully for other seedlings? Temps and lighting duration sound good.

I made my own last year from bulk since I had about 150 gallons of fabric grow bags to fill. Roughly 3-3-1 peat-compost-perlite. This off-season, I've been emptying those a few at a time into a trash can to refresh it for this season. I wanted to fluff it up a little more so I did add some fine pine shavings to it. I've since learned that probably wasn't a good idea as it's uncomposted. It wasn't all that much though (roughly 2-3% by volume). It looks like more in that pot, and maybe that one happened to end up with more. But I think that picture exaggerates it some as they kinda got scraped to the top when planting the seeds.

I also mulched the top of each grow bag last season, and even though I tried to scrape most off the top when emptying the bags, some got mixed in.

I think it is just something with the soil in that pot. The other 3 pots of spinach seed planted at the same time look pretty good today. And the tomato seedlings I mention yesterday that looked kinda shriveled look much better today. I guess they just hadn't opened up yet. Now the leaves are starting to unfold. It's only the 2nd year I've been doing seed starts so I'm still learning what everything is supposed to look like.

I may also have to rethink using the same soil mix for my indoor pots vs my outdoor grow bags. I was hoping to get by using the same for both, but I may need to maintain a separate (better) mix for the indoor pots. I do microwave it first before using it in indoor pots though.

Sounds to me like you're doing everything right. I guess you have a one-pot mystery on your hands.