Assuming you have checked the tune and the engine is not too rich or lean....
How old is the fuel? In the US, they put ethanol in most of our gas, which means it only takes about 2 weeks to absorb water from the atmosphere and cause all kinds of problems.
I'd also suspect the spark plug. NGK seems to give less trouble than anything else in model engines, and also make sure it's gapped properly.
After that, a leaky reed valve (if it has one), air leaks pretty much anywhere, a bad diaphram or pump, or an inadequate ignition battery could all cause your problem.