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?
Edge sealing vacuum machines like yours are not really designed to cope with liquids, either water or oil. In fact, you can permanently damage your device if those liquids get sucked into the pump. There are ways around this, one is to feeze any liquids you wish to add to the vacuum bag or if your model supports it you can follow the advice from eGullet here:
Sealing liquids is possible with clamp-type machines. Other members stated this long ago. I did not manage to do so at first, but now I have learned the trick. It does not work with fully-automated machines that have no "seal"-button and no external vacuum-port.
Place the machine in a way that the bag can hang down vertically. Place the adapter on the external vacuum-port (without the tubing). When you start vacuuming, air will enter through the external vacuum-port, so almost no vacuum is built up. By closing the port with your finger, vacuum will rise and so will the liquid in the bag. Before the liquid approaches the sealing bar, reduce vacuum by lifting your finger, and press the seal button. Make a second and eventually third seal in case the first one should not be perfectly tight.
For small amounts of liquid in the bag, you could include a folded paper towel across the opening and just below the place where the seal would be. The paper towel will absorb moisture and allow the bag to seal.
Alternately, you could use zip lock bags when cooking low temp, in a water bath. Use the displacement method (submerge the bag in water and seal just as the air leaves and the liquid gets to the seal).