Replying to Why do my posters fall!??

Okay so I have some ski posters, and other poster that i put up. I did it first with masking tape, i just rolled it up and made it double sided and put it on. Next morning all my poster fell  everywhere. And next time i tried duck tape. worked better but after a few days they fell agaiN!!!!! what the hell? i mean i shouldnt have  to fill the entire back of the poster with tape, i mean a few (about 6) should be enough for a small poster. IM REALLLY CONFUSED!