The little one may need more time on a heating pad. For my past few grows, I've kept the seedlings on a heating pad (after they pop up, I put the plug in a leca-filled net pot and put that on top of the heating pad). I also water them very carefully during that period with a syringe, a few times a day, because the heating pad dries the moisture faster. This has produced faster, stronger growth of seedlings for me.
Prior to this technique, it would take much, MUCH longer for them to grow. They needed some more heat.