My door mounts seemed to slide on just fine. There might be a burr or something where the allen screw comes out. Don't forget you have to slide them on from the top end of the strut prior to mounting the strut into the retract body.
I used the allen set screws to hold them on, but some of the stock screws were binding in the holes. By using some bit longer ones I had laying around I was able to get them to thread in better.