Here's a few suggestions to eliminate potential causes of vacuum bag seal failures.
Use bags larger than you think you need so you have ample flat edges for sealing, particularly with the large pieces of meat you are cooking.
Fold the bag's opening back over itself forming a cuff well beyond where you plan to seal. The plastic must be absolutely pristine to achieve a perfect seal. Don't touch it there either. Seal twice a few mm apart at both ends.
Consider the cost of wasted food and buy the best quality SV material you can.
After the first hour of a cook inspect the bags.