Garam means warm, and the spices in it (cardamom, clove, cinnamon, pepper) carry their flavour in oils that boil away quickly. Twenty minutes in a bubbling gravy and most of what you paid for has left through the kitchen window.
The rule
Build the gravy with the sturdy powders: coriander, cumin, turmeric and chilli. Add garam masala in the last two minutes of cooking, or off the heat altogether, and cover the pan.
How much
For a curry that serves four, half a teaspoon of a fresh blend is plenty. If you are used to using a tablespoon, the blend was probably stale or bulked out with coriander.
Smell the tin before you measure. If it does not make you want to cook, it will not improve in the pan.
A simple test
Make a plain dal. Split it between two bowls. Stir a pinch of garam masala into one and leave the other alone. Taste them side by side after two minutes. That is the whole argument.