I just picked up a Hi Power Mark 3, and I can't get it reassembled. I had no problem taking it apart so I could clean it. I got the spring and barrel on without a problem. I put the slide on, pull it all the way back with the safety to secure it where I put the retaining pin in. The pin goes in, and it goes through to the other side but it doesn't lock in. No matter how much pressure I use, I can't push the pin in any more so it locks in. When I shift the slide a bit, the pin will fall out. Does anyone have any ideas? Thank you.