When I had my 16 I used to have an issue when solo stepping the mast that sounds similar to yours except that I didn't need to strong arm it back into place. A nice push would do it. I found that if I had someone available I would have them hold down the mast as I stepped it. This would make the mast rotate at the proper point and it wouldn't hang up.

Otherwise I would check that you have a mast step and mast base that are compatible or get a new step link as previous posts suggested.