Gears don't go in after engine rebuild

[JBK]

Hi, I have a very frustrating and confusing problem with my fully rebuilt 2013 EC300. I'm 99% sure that my transmission is built correctly since I never took it completely apart, only the first gear from both axles. So, the problem is that it's most of the time very hard to put gears in. It at least 50%of the times I shift goes to neutral before getting the gear in. And shifting down is even more painful, it just goes to neutral when I pull the shift lever and it's really hard to get it to shift down.
This causes the problem that I can't really ride at all because I never know whether the next gear goes in or not. And sometimes it makes awful noise when i try to shift down, it sounds like it does the shift but then it only goes to neutral
I would be very very very thankful if someone knew a solution for this problem. I hope the explanation was good enough for you to understand.