We are closing our pool. We blew the lines and plugged the bottom of the skimmer and the two return lines to the pool. On our final plug which is a purge plug, we put it in blew the line and after about a minute the plug pops out from the pressure. We tried several more times. (blowing the line longer, blowing the line less, pusing the pin to releave some pressure, blowing the line with less psi) It is a new plug any suggestions to our problem?