vacu vin question

[newsguy]

on four occasions in the past six weeks or so, i've gone to reopen a bottle that had recently been sealed using a vacu vin stopper, only to find that the seal was broken and the bottle was full of air. this has happened on bottles that were sealed nearly a week ago and ones sealed the night before.

am i pumping too much or doing something else wrong? or do these rubber sealers wear out and need to be replaced? i bought 3 of them a little more than a year ago, and typically have only one in use at any given time (so it's not exactly as if they're getting unusual wear and tear).