I?m not familiar with a dps lower unit. But recently had a similar problem reinstalling my alpha. Could slide the drive in and out but it would not seat in the last inch. I had had the motor out replaced the gimbal and drive cupler so was worried about alignment. Borrowed an alignment tool and everything checked out. When removing my drive you had to set shifter to full forward position. What ended up being the problem was that the shift fork on the drive had turned sideways and was not slotting into the shift tab on the transom assembly. I couldn?t see it happening because it is up underneath in the back and I was working alone. You have to make sure the drive side is set right and not off by 180 or your gear selector could be reversed. Not sure if this applies to you but I figured I?d try to help. Good luck.
Another trick you might want to try. Put the lower unit in forward gear and rotate the prop clockwise when pushing up on lower unit it should seat all the way then place a couple of bolts in to help hold it.
