If all the inserts stop about the same location and fit fine in your other vehicle receivers then you don’t want to grind on them, that’s not the problem. The problem is on the inside of the receiver they will not go into, most likely rust, dirt, rocks, mud or a combination those are the problem. You need to clean out the inside of the receiver, as stated above a good file and work all four sides of the receiver from the front all the way to the back, you may even wash it out with a pressure washer then use the file. Once you have it sliding in and out freely take some paint or even primer and spray the inside to help keep from rusting.