I have had this problem with a cheap internal door - the screws pull out of the hollow pressed hardboard - precious little material for the screw to grip.
I solved mine but I'm sure it's a temporary fix by drilling out the holes in the metal handle and using bigger diameter screws - it will not be long I'm sure before I have to invest in a better quality solid door rather than a hollow one.
Good luck...
Peccavi
November 2009