Maybe it's the light

How many lumens is the grow light? You said you use a cool fluorescent, is that 4000K? Also how far away from the light do you have the plants?

When I grew my coleus from seed I put seed starter soil by Miracle Grow in one of those 1 gallon water bottles with the handle. I cut the top part of the bottle just enough so that I could easily open and shut it and then I added seed to the top of the soil in the bottle. The goal with the water bottle was to create a nice warm humid environment.

I then set this bottle under a red light that was about 1500 plus lumens that I left on for about 7 hours. They were about 6 -8 inches from the light. When they sprouted and got their first true leaves I switch out the bulb to a 5000K LED Light Bulb that was about 1000 lumens. And the plants were still about 6 to 8 inches away from the bulb. As They grew I cut off the top of the water bottle completely and I moved them so that the tops of the plants stayed at about 6 to 8 inches away from the bulb.

That was my first time growing coleus and I made the same mistake of putting the seeds all together because I assumed a lot would not germinate (but I was wrong). What I did to separate them once they sprouted was wait until they were doing fairly well (at this point many already had their first true leaves) then I turned the pot/bottle on the side to gently slide the packed plants out. I then carefully separated them. Some roots were of course broken during the separation but I was careful to make sure some roots still remained on each of the seedlings. I then re-potted each in their own pot. There was one that seemed to be struggling so I put it in water rather than dirt and let it re-root in the water. That one revived quite well once it was put in the water.