I use a two-piece system, Coloplast Mio system. The wafer is changed once a week, but the bag is 3-4 days depending on if the charcoal filter gets wet. Also, the bag will break down over time from the output wearing through the plastic.
Check if there is a filter in the bag near the top. If the charcoal filter gets wet, it will allow smells to get out. If a quick movement or cough causing your stomach to flex allows gas to escape, you might be having an issue of it not sealed to the skin properly.
A rare problem that I have had once was a manufacturer defect where the skin seal material and the harder plastic part on the two-piece separated but would not show unless you look at it closely using a mirror to find the separation.
During the winter time, warm up the adhesive before you apply it. Sometimes if it is too cold, the adhesive will not bond as well as it is designed.
Hope some of this helps.