I had a .243 that was having the same problem. I cleaned the latch and that helped, but the more I shot, the worse it seemed to get.
I think I was contributing to my problem. At the time, I was new to reloading and was not checking the length of the cartridge. Over time, they were stretching and, as a result, they were beginning to (ever-so-slightly!!) protrude out of the barrel and interfering with the proper locking of the barrel. Once I re-sized the cartridges, the problem disappeared.
Once I corrected that problem (and the oil on the latch) everything was fine.