It could be several different causes, water temp, not enough borax, incompatable paint....My last attempt , with Humbrol paints,& what I thought was a right borax/water mixture, turned out horribly.Satin white base coat, colors I have used before, etc, it should have worked, but...no.
I think my biggest problem was not the wrong WATER TEMP(76F), but I did it in bright sunlight, and the sunlight caused it to dry prematurely.
Maybe.
It's still quite a hit'rmiss kind of thing, I'm going to try some Krylon brush on paint I saw on another post here, I picked up 12-13 bottles at a yard sale a few months ago, all for a dollar! At least it's worth a try, and cheap too !