Often it's infection, so they get blocked by the pus; sometimes they also get blocked by tooth root overgrowth, in which case the blockage can be permanent. Even if it is an infection, it's often secondary to something else, such as tooth root overgrowth, or irritation due to use of sawdust/shavings etc. Santa had chronic snuffles so used to get lots of pus blocking hers occasionally, she had hers flushed under GA because she was too wiggly to do it conscious, but sometimes it can be done conscious if the bun allows. I seem to recall I paid about £57 for a flush under GA.