When you transplanted them, did you bury them up to the cotyldons?

Could they have been watered too much? How often are you watering them?

Yes I did plant up to the cotyldons. I water them when they are dry so it's not too much water. They are normally in a greenhouse except for lately when it's sunny I've been putting them out for the day but put them back at night because of possible frost. So I just don't get it. I've never had problems before.

A picture might help if you can post one.

Questions: How old and big (inches tall) are these seedlings? Even though stems are thin, are plants around the size as this time last yr? Just thin stemmed, or what you would also consider lanky? What if anything did you do differently? Different potting mix, more (maybe too much) or less fertilizer?

You indicated you are not overwatering. Still, do you see any signs of damping off at the base? Also, what do the roots look like, healthy and white? A bad potting mix can sometimes cause stem or root rots even if you are not overwatering.

Since you indicated the seedlings actually fall over, potting up deeply might help salvage the situation, with root ball set all the way to the bottom of the new pot. That is what I would try, anyway.

This might be problematic of course, if your plan was to sell them in 6 packs (?)

Stems will strengthen if put in some light wind. I use a fan directed at the plants. They will strengthen. You might repot them again in the 6 packs, but planted deep again. But the fam should work.