Trans wont shift

Check the basics first, check the fluid level in the transmission if the fluid level is OK and doesn't smell burned or look dirty the problem is likely to be in the transmission. Has the fluid be changed periodically?

I recently had the same problem with my son's 1990 200 turbo. We rebuilt the entire trans and still had the same problem. Short story: We found debri plugging the only metering hole in the governor causing it to not shift until the trans pressure was really high. He is now a happy son.