1982 cm250 slipping gears, and not getting the right speed out of each gear?

I have a 1982 honda cm250 bike. I recently had the motor rebuilt with new shift forks, etc. I got the bike back and it's doing this issue ( the same issue as when I took it into the shop). I was told that these are common issues with that year motor. I'm having a hard time believing this, as people say it is a great beginner bike.

It's doing a couple things. 1) it doesn't want to shift from first to second. I hit the shifter and it stays in neutral, 2) I'm only getting 10-15mph out of first and second gear. Before it sounds like it red lining.

What do you mean by slipping gears?

If a mechanic did the work they should stand by it, if they couldn't fix it they should never have taken the work on, or warned you it might not fix the problem.

Clutch is not adjusted properly. Take it back to the shop who worked on it and ask for help.

Slipping is usually caused by worn clutch plates but if you just had the transmission rebuilt you really should take it back to the shop that did the work.