One thing to consider is that embossing powder (EP) has a shelf life - they don't last forever. Be sure to shake your EP before using as the particles will settle so you want to make sure it's mixed well.
As for the shelf life, if you are buying at thrift stores the EP may no longer be good. I had a problem a few years back and after some research I found that I only had problems with old EP. I bought a new jar and it embossed perfectly.
Just a thought - may not be your issue, but see if a new jar works better.