Does capsaicin lose potency with oxidation? I have a packet of ground garam masala and if I make a stew with it it tastes very bland and has no taste.  Two of the ingredients in the garam masala include pimento and cinamom which I know contain lesser amounts of capsaicin.

However even so I cannot taste any heat in the stew.  Is this because the capsaicin content is too low or does the capsaicin lose potency with oxidation and time?

The problem with saying yes is that most chilli powders seems to be hot even after they have been oxidised over time.
Garam masala is not a "hot" (piquant) spice mixture. It doesn't taste spicy because it's not supposed to. I'm not sure if cinnamon contains capsaicin, but it certainly doesn't contain a lot; otherwise it would taste hot, like cayenne does.

I suggest that you try to think less about chemicals and more about tastes. Taste some garam masala; that's what (your) garam masala tastes like. (The taste will change somewhat during cooking, but it's not a night-and-day difference.) If you're going for heat, taste other spices until you find one that's what you're looking for.