Occasional leak from drain of fiberglass shower unit.
I have a fiberglass shower unit on 2nd floor of my home. Shower only, not a tub. Whenever I am away from home for about 4 days or more, when I take the first few showers after arriving home, it leaks profusely into kitchen below. My retired plumber friend takes drain apart and re- puttys it and it is fine, until the next trip away from home. This has occurred about six times so the putty fix is not the answer. What can we do for permanent fix? I have a drop ceiling below the shower so we can get to the underneath if necessary.