Places that the drive gets stuck:
1. Prior to entering coupler.
--Can happen if the driveshaft isn't parallel to axis of the line from the gimbal to the coupler. When inserting the drive, make sure everything is on the same plane. Distance between the drive and bellhousing should be the same at all 4 corners as you're pushing it on. Sometimes rotating the shaft 90 degrees helps, as the u joints change orientation and the shaft sits at a different angle.
--Splines in coupler/driveshaft aren't aligned. Push drive on while turning prop CCW. Doesn't take much of a turn.
2. Shift shaft and shoe aren't aligned/something bent. Can happen if in neutral/reverse when trying to remove/install drive. You can look behind the drive when it's stuck to make sure they're engaged correctly.
3. Drive shaft O-rings. These take VERY little grease to get on there. Too much will get it stuck. At this point the studs are flush with/just past the drive
4. Too much grease in coupler, OR foreign body (old o-rings?). Double check the area inside the coupler.
5. Gimbal out of line (or incorrect installation in your case). As mentioned, whack it. If the alignment tool goes in smoothly and fully, this isn't the issue (the alignment part. Could still have a faulty installation I guess).