In addition to the hammer block (and it needs to be slid up on the rebound slide pin) make sure you start the top of the side plate along with the front by the yoke screw both at the same time. Don't do one before the other.

I have encountered a few that were fit extremely tight by the factory, but even those you could press it at least 75% of the way home with just thumb pressure. Don't use anything else. Stop beating on it with your hammer handle!!!