Does capsaicin lose potency with oxidation?
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.