The water in my shower doesn't drain properly, so I used the plunger and the problem seemed to be solved. However, the next time someone uses the shower it's the same problem again: water doesn't drain, plunger, water drains.

I wonder what the problem is, because the pipe seems to be clean, otherwise the water wouldn't drain as it does (after using the plunger). But why doesn't the water drain the next time? I was wondering if it has something to do with air in the pipe. But even if this were the case, I wouldn't have any idea how to solve the problem. I'm grateful for any suggestions.