ORIGINAL: Turbulence
Let me tell you a little story. My RAM 500 was working fine, did two flights then went to go for the third and final time of the day, got my new can of Powermax and No start, filled it up again, nothing. Squeezed the trigger into the air a couple of times and it would smell like gas, but I would not see a large burst so I figured it was the triger valve. threw it on the trash and ordered two more from BVM, filled up the plane again and nothing. went to BIG 5 for some basketball stuff and found 5 cans of Powermax, so I bought them all, just for kicks I tried a new Can of powermax and BAM problem solved. End of Story
Now it does not happen too often, but you might also have a bad can of Proane (in my case, powermax) I checked my trash can, trigger valve was already gone. I had also tried one of those Jet Cat style nipples and nothing would come out out of the can. Darn had I known, anyway if your settings are right, try that too before you go Crazy
Turbulence
The Powermax cans have a clunk inside them which is why you are able to get liquid fuel from out of them with the can in the upright position. I have had several cans of Powermax that have had the clunk line come loose inside the can.
When this happens you are still able to get gas out of it, and unless you have a clear line going to your onboard tank (so you can see the liquid and bubbles moving) you will not notice anything abnormal when filling your tank, but since there is no liquid in the onboard tank there will not be enough gas in the plane to get the engine started.
If you do find that a Powermax clunk has come off the can is still useable, but now you will just have to remember to turn the can upside down when filling the airplane.