Hi, it could have many causes, so you have a few meters, lower the lamp a bit in intensity or hang it higher. I also started with the Spider Farmer at 40% about 55-60 cm away, then adjusted it again when they got bigger.
How's the humidity? When they're this small, they take in more moisture through the leaves than through the roots, so if you give them too much water, it can harm them. For seedlings, just put a plastic cup on top so they stay moister, just not too moist to avoid mold, and lift the cup occasionally.
If the soil is too compacted, then it grows very slowly as well.
But it could also be the genetics; maybe it just needs a bit longer and is busy with the roots.
And finally, due to disease, but it doesn't look like that with yours. If I were you, I would give it a few more days, and if nothing happens then, get rid of it and plant a new one. It's a shame, but sometimes it's better. Good luck!