Troubleshooting 7a indoor seedlings

Hi All,
Looking for some suggestions - I started indoor seedlings about four weeks ago (based off instructions from susan's in the garden - the best! as well as seed packet instructions)... Added small amount of sphganum moss to top, most germinated really quickly and started off strong. Added small amount of very diluted fish fertilizer once true leaves grew in... 1-2 weeks later and it seems the seedlings have pretty much stopped growing and leaves are starting to turn yellowish. Not sure what I've done wrong. Didn't appear crowded in their cells and roots weren't growing out but I up-potted today anyways thinking they might thrive more in a larger pot? Any suggestions appreciated, I'm a newbie. Thanks!