Hi.
From my 7 to 8 years experience, heat always seems to break the bond (in prolonged conditions). My suggestion is whilst enjoying a hot shower, turn the temp down a little before you wash the unit. That always does the trick for me. On the rare occasion that it does come a little loose I normally dry the hair, and the adhesive becomes tacky again. Generally speaking if you clean your scalp properly and not leave the unit on for more that 3 weeks (for full bond) you should have no problems.
Hope that helps