The charcoal canister has a couple of check valves on it. The fuel vapor has to build enough pressure to push a check valve open before it can flow into the charcoal. The pressure required to open the valve is supposed to be pretty low but it exceeds what you can produce by blowing on it with your mouth. I think the valves get sticky over time and the cracking pressure can go up.
'03 KTM 450EXC
"If you know what you're doing, then you aren't learning anything." - A. Lincoln