Box locks have so few parts, nothing serious usually goes wrong.

I have seen 4 guns that had problems with one barrel firing Remington primers, but not with Winchester primers. The Remington's have harder cups.

The other problem with an English box lock was a blanked primer disk stopping the hammer from falling completely. Flushing the action out with Remoil removed the disk. I got the gun for a song because, "it doesn't work!"