could it be the wrong offset ???? Adding a small spacer would fix the issue but it would move the wheel further out.....
Does the wheel fit correctly other than the cap popping off ??? How much interference is there ? Is it more like .020" or 1/4" ???? If it's only a few thou then take a grinder to the center stud and grind it down a little so that it's not hitting the cap....
Wonder if the hubs were ever changed on your car?? some shim left out maybe?? aftermarket hubs?? I"d take a grinder to the end of bolt, as suggested above, unless it's really gross amount of metal and into that cotter pin......:clobbered: