How Long Does a Central Incisor Crown Last?

I have had 2 central incisors prepped and crowns place approximately 20 years ago. One crown came loose last year, was cleaned and recemented. It has now come loose again, and I am concerned. I am going back to the dentist (not the dentist who did the original work), to have it recemented again. What could cause this problem? I am not a dentist, but could it be that mouth and gum changes have created a poor fit? I am in excellent health, floss 2x a day/brush/visit my dentist bi-annually. Thank you.