Is it possible to start some new seeds in Jiffy Mix? Your plants sound stunted. At 5 weeks they should be much bigger. Have you added any fertilizer? Are your containers draining? Not sure what the problem could be.....I've heard that Pro-Mix is pretty good stuff. Can you test the Ph? A photo might help.

I had some curious problems with year with some stunted tomato seedlings. I'm using my own starting soil. I understand that overly wet soil can cause stunting. If the soil is saturated with water, it can interfere with uptake of nutrients. Not quite sure how that works, but that's what they say. I think it's called "waterlogging", and has to do partly with limited oxygen availability. It will be interesting if potting up into potting mix fixes it. Pro-Mix is good stuff, but maybe some care needs to be taken when using it.

I just tested the Ph and it's just over 7, but my meter is a cheapie, so who knows if it's accurate. I think the issue is starting out with too wet of mix, then possibly compressing it too much and it never really drying out. I guess there's a learning curve with the added "wetting ingredient"? Edited: I just read a few different growing forums where folks have had drainage issues with Pro-Mix. The company website has good info on compacted medium, which must be my primary issue, moisture retention/compaction:

https://www.pthorticulture.com/en/training-center/how-to-avoid-problems-with-growing-medium-compaction/

I use Pro Mix with no problem. Don't pack it down too hard in the pot. Fertilize with half strength Miracle Gro with every watering. Bottom water.

I also think that maybe the stuff lacks nutrients. After the seeds germinate and grow couple of leaves, then they have already used up the nutrients in the seed and need food.

The easiest way os to make a half strength solution, using MG water soluable fertilizer.

BTW. My 5 week old seedlings are mostly about 10 inch tall and getting hardened off

There are all kinds of pro mix and all have different qualities. I used to use pro mix HP for starting plants but found that it dried out too fast because it's a fast draining mix. Not very good for my greenhouse where plants tend to dry out faster. It is fine for plants that don't like to be wet but not for your average plant that likes to be kept moist I switched to pro mix BX and have had great success with it It has better water retention without water logging your plants.

I use pro mix all purpose mix for pots and baskets because it contains a 4 month slow release fertilizer. All pro mix contains Mycorrhizae a hormone that makes for better roots and is my choice for growing

I agree with dowlinggram. HP stands for High Porosity and is intended more for container gardening of mature plants that don't like wet feet. I have no idea how it would do as a seed starting medium but apparently not very good.

How much did you pack it down in the cell packs/pots that you started the seeds in? I just gently tap the cell packs on my potting table after filling and then add whatever I need to get them back to the top. I don't pack it down at all. Packed soil would cause the plants to lack oxygen and fail to thrive like you are describing.

I've always used Promix BX with Mycorrhizae and Bio-fungicide as my starting medium and have never had a problem. I also fertilize with 1/4 strength MG liquafeed Tomato and Veggie formula (9-4-9) at every watering. Promix BX does not have any added nutrients. I keep my seedlings pretty wet when in cell packs as I don't want them to dry out with a fan running on them while I'm at work.

I think you are on the right track trying to pot them up using a different medium at this point. Good luck!

Yes, I did pack it down a little too much. The potted up starts seem to slowly be getting bigger now that they are in potting mix. As a backup, I sowed everything all over again in Jiffy Mix four days ago and they have all come up. If everything grows, I will be drowning in tomato plants! Good problem to have. :)

in the future, you may want to consider using coco coir. it is superior to peat, and yes, that includes pro mix. it does not need any additional amendments except nutrients. Walmart has organic 8lb bricks for under $8. enough seed starting mix to fill an entire wheel barrel and plants (especially seedlings) love it because it's so airy and allows a lot of oxygen to the roots, even when wet. good moisture retention and stays very loose..

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5539188/

just a recent study on coco vs peat for the skeptics ;-)

TLDR study conclusion:

CC = Coco

RC = Rockwool

PVC = peat/vermiculite

Coconut coir was a potential substrate that could be widely used in tomato production. Compared with RC, CC showed higher K and S uptake by crops, photosynthesis, individual fruit weight and total fruit yield, and lower uncredited nutrient (the lower, the better). Compared with PVC, CC showed higher P and K uptake by crops and total fruit yield, and lower uncredited P and K. CC did not influence BER compared to RC or PVC. In addition, effects of substrates on fruit quality were generally not obvious.