I admit because ziploc bags are o2 permiable the don't hold a hard vaccum well. The best way is to double bag. Also if you put a small amount of vegatable oi [a single drop usually does it] l in the sealin strip it will hold longer. Also a big part of the problem is getting a good solid seal on the bag itself. Often when closing the bag the seal is only partially closed. I have found this when playing with them in a reverse situation making an air bubble. It appears closed but pops open with little encouragement after a short time.