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? 
As far as I am aware, you cannot recognize this in advance.   What you describe is due to very damaged cell structure in the fish. The "water" are the fluids contained in and around the fish cells, which make the filets juicy. They flow out when the cell walls in the fish rupture.   The reason for rupturing is that the fluids are water-based, and water expands in the 0 to -4 Celsius range. When meat or fish is flash-frozen, it goes very quickly to under -4, and in the small time it spends in the problem range, the cell walls withstand the pressure from the expanding ice crystals. When it is frozen in a "slow" process, the cell walls rupture. Or, if it was flash-frozen, but sometime during storage it spend long periods above -4, it will also have this problem.   There is no way for you to recognize whether a piece of frozen fish or meat in the supermarket was flash-frozen or not. So sorry, but you have to rely on luck, and maybe try to find if there is a correlation between certain brands and quality by buying them repeatedly.  