They seem to be germinating in the wrong season... I guess you shouldn't let them go under O°C.
try to give them as long a growing cycle as possible.
I've got the same problem with a seedling too. it germinated after I'd reworked the soil in the old seed pot. what's more, it looks like a species I don't have yet... I don't really know what to do with it.
It cohabits with Viola walteri, which I'm also fond of.