Why does garam masala powder taste bland?
I think we’re up against this mis-interpretation of the word ‘sweet’ again. 

Garam masala is a blend of aromatics you add towards the end of cooking. Many of the ingredients are the same as a generic curry powder, but emphasis on aromatics, so you get some elements of those spices in a long cook plus a quick burst of new aroma. 

Its shelf life is relatively short, maybe a year, max, once opened.
It’s a simple smell test when you open the jar, the aroma should be huge. As it gets old, the last smell to go will probably be the cloves, the rest just fades to a generic ‘musty’.