I am assuming you mean dirty or fouled & harder to load, not physically tighter - as that would be impossible.
: You may simply need more or a different type of lube. With the proper lube, you can shoot for hours on the trail-walk, perhaps 40 to 80 shots, without having to swab the bore. We clean or swab the bores only after the day's shootingis done. We normally use spit for lube, as each charge is loaded at each station jsut beofre firing so they don't sit for extended periods of time, loaded with a spit-wet patch. That would or could pitting in the breech area.
; For hunting, a different type of lube is required, and depends somewhat on temperature as well, as some don't work well in freezing weather.
: What patch material and thickness are you using along with ball size and alloy?