The only two problems I have heard about with the transmission not wanting to shift, is a broken pawl spring in the shift mechanism, but if it breaks, it won't start working again .
The other is a problem that I had, the oil seal that the shifter shaft goes through, was put in crooked and allowed you to upshift, but when you wanted to down shift, fourth gear was the lowest you could go and again, it didn't 'fix' itself with waiting 10 minutes .
All I can suggest, is the usual things, correct oil level in the transmission, look the shifter and clutch mechanisms and see if you can find anything out of the ordinary with them .