I had the same problem with my 89 CR250 several years ago. I tried stretching the spring that pushes axially on the gear, and even using two springs. I even streched the one spring, heated it up and reinstalled it. I never could fix the problem, and must have had the side case off 15 times trying to fix it. I think either the gear or shaft, or both must have been worn. I eventually sold the bike. I had to finesse the kickstarter at the top of the stroke until I felt it catch, then give it a kick. Sorry I could not be more help.
Danny