I've owned this Browning Buckmark for many years, and have always been impressed with the accuracy and ease of use of this very inexpensive gun. But recently I have a problem with the slide being very stiff, meaning I barely have the strength to pull it back to chamber a round and cock the hammer. I've disassembled and clearned and lubricated the slide, checked the spring, etc. and everything seems fine but then when fully put back together, it is still very difficult to pull back. Any thoughts?