I am occasionally using my vacuum machine to pack bags for sous vide cooking. The model is the FuturaVac clamp-style table model (see e.g. http://www.sousvidenorge.no/produkt/sous-vide/vakuum/futuravac-bordmodell)
Recently, I have experienced that the sealing of the plastic bag does not function as it ideally should. This morning, for example, I was packing three frozen chicken breasts. They had thawed a little in the fridge, but still had some ice crystals on them. I added some oil to the bags and vacuumed them using the machine. The seal for two bags was not 100%. I even tested it by packing one bag within another, and I could see liquid (oil) moving up towards the machine in the innermost bag, which it should not had the sealing worked as it should.
My question is this: How can I avoid this poor sealing? Are there any tricks one can use?