I suspect your seedlings need nutrition.
Let me explain in a little more detail as to my reasons why...
I’m not familiar with “Pro-Mix seed starting mix” (seems that it is a US product), so I looked up the company website for the product. It claims...
Organic fertilizer that gradually feeds plants for up to 3 months
Once seedlings progress beyond their cotyledon stage - that is, they commence growing leaves other than the first two leaves that are contained in each seed - they have exhausted their seed-based food supply and require nutrition from their growing environment.
The seed starting mix contains an organic fertiliser that is claimed to provide food for up to three months, but we don’t know the amount of fertiliser or actually how long this will last.
As most seedlings in most human-controlled growing situations do not remain in their original sprouting media for long, it seems most likely that the included fertiliser is adequate to provide a little nutrition for sparsely planted seedlings.
Your planting however is very intensive - a lot of seedlings growing very close together. While this is not the issue, it’s important to recognise that these seedlings require the same nutrition (or actually more due to competition) as if they were spread out in a larger area.
I suspect your seedlings have exhausted the fertiliser contained in the original growing media and require an additional level of care that now includes fertiliser as well as water.
To your watering can, once a fortnight, at half the manufacturers recommended dosage, add:
- liquid soil bacteria to inoculate the soil with beneficial microbiology;
- liquid seaweed extract to feed the soil bacteria;
- liquid organic fertiliser to feed the plants.
I have favourite products that I use regularly. While it is unlikely these are available in the USA, there will be similar products available in your area.
(As an aside, I note that worm castings - “vermicaste” - is an excellent alternative to this recipe, however I’m making an assumption that, as a new gardener, you don’t have access to a worm farm.)
Any questions feel free to ask in the comments and I’ll update my answer.