I had one tap in my shower getting very tight and dripping as well. When I opened it up to replace the seat I lubed the threads with plumbers grease and replaced the o ring with a spare I had on hand. Seemed like through the years as the O ring leaked the thread got messed up. Not sure and normally would replace the entire cartridge, but in this case I have more work to do in the wall. The grease made it feel like a new tap again.