Several reasons ;-
Assuming it runs PROPERLY once started, do the following ;-
1) Change the plug (even if it's newish)
2) Use FRESH gas/oil (less than 30 days old)
3) Check primer (if fitted) is filling the fuel lines. If not, replace. If it is, or there is none fitted, replace the carb. diaphragm.
Phil Saunders, November 2007