I have half a dozen of these vacu-sacs bought from Lakeland couple of years ago. Although, following instructions, they work initially, the vacuum seems to go and they slowly reinflate over next few hours, defeating the object totally! Also tried new cheap ones from Lidl recently that do the same. Am I going wrong somewhere, or have others had this problem? Trying desperately to pack away dd's outgrown baby clothes and dh's hoarded shirts...