Yes, it is, and what you have described is exactly what I was doing. Three minute rattle (or more) agitate and up end. Inverted cleaning blast, but no dice.
I've gone to the extent of solvent cleaning out the nozzle after the upside downing didn't clear it. I played with finger position on the nozzle and what seems to happen is that after a brief good spray, it sputters and spurts then i get a blast of seemingly straight propellant.

The Poly itself seems good, just the can is not behaving.