Sous Vide bags losing vacuum

Hello fellow Sous Viders - My problem concerns maintaining the seal in the plastic food pouch. I have the seal-a-meal machine for vac sealing, and use a Tower slow cooker / Auber instruments P.I.D temp controller set up for cooking.

I first tried out the vac sealer on a pack of bacon - which I then froze. However, upon defrosting the bacon at a later date the vacuum appeared to be lost and the plastic film was no longer clinging tightly to the food.

I have also had a problem during the cooking of a brisket joint. When checking the water level in the pot I noticed that, once again, the bag appeared to have lost it's vacuum - although the cooking water was clear and didn't appear to have mingled with the bag contents.

Am I actually losing the integrity of the bag seal here? or is this just some function of expansion / contraction that gives the impression of seal failure?

many thanks for any input,

Pete