Two possible reasons. One, the bags were not sealed properly by you or had small holes in them. The other reason, that ziplocs are not meant to hold a tight vacuum seal is most likely the actual reason. You can get a lot of air out of them, but they are not guaranteed to be airtight. Some of them are really good at staying sealed even so, I might try a different brand of bags to see if they work better then the ones you used.