Sous vide bags inflating

I know it has been posted before, but I'm still not having any luck...

ANY time I try a long sous vide cook (more than a day or so) I end up with at least 1 hyper-inflated foul smelling bag (usually more than 1).

Some details... I'm using a sous vide supreme. I cook at 140-145 and have verified this temperature with several thermometers. Food (oxtail or short ribs) is sealed in "seal a meal" bags (the seals always hold, thankfully...), I have tried searing first, boiling after sealing, freezing(then thawing in the refrigerator...), additives to the bag (wine, broth, herbs, etc) or just the meat alone (sealed in a chamber sealer if liquids are used). I have compared "large batch" (3-4 bags) and single bags (1 layer of meat, pieces touching but not overlapping). Sometimes the bags are set flat in the chamber, sometimes I use the rack. The bags are always completely submerged. But the results are always the same -- in a multiple bag batch I am lucky if 1/4 DONT inflate. My recent trial of 1 bag (oxtail at 145) has inflated this AM (60 hours into cooking, although I could see some gas forming last night...).
I would usually throw in the towel with this kind of success rate - but the bags that DO work are just too amazing to give up on. Nonetheless, I am getting tired of throwing away money like this. What am I missing??