See this alot. Generally, the reason is there is still a load against the gears. And you have to really force it to get the gears to shift. The solution is generally you have to back up/go forward a little to release the tension. My stock case will do the same thing, and I just got to release the tension, and it will shift very easily.
This!
It's because of the binding between the front and rear drive shafts through the case which binds the gears together very tightly. Usually if you get a slow roll going and step on the clutch, rocking the steering a little from side to side will unbind the drivetrain enough for the shift to come easy. It's not as uncommon as you might think. Just don't force it! You already see what happens when you do.
