@Susan3495 Sounds to me like these guests are scamming you. They report a clogged-up shower, yet don't want it fixed, but right away say they want a partial refund, even though they have another shower at theiir disposal. Unh-unh. As others have advised, insist on going over yourself to check it out and if a plumber is needed, hire one.
Airbnb stated protocol is that if a guest has a issue, they first notify the host and give the host an opportunity to correct it. That doesn't mean reporting a clogged shower and then refusing to let you have it remedied and asking for money back instead.