DIAGNOSIS That's usually a sign of low compression, or the starter mechanism itself is gummed up. On these, the recoil spring can also bind if it's misaligned.
FIX First, remove the starter cover and inspect the recoil assembly. Clean out any sawdust or debris. Make sure the spring is seated correctly in its housing. A little light grease on the spring can help. While you're in there, check the condition of the pull cord—if it's frayed, replace it before it snaps.
IF THAT DIDN'T WORK The engine might have low compression due to a worn piston or cylinder. You can test this with a compression gauge (should be over 110 psi), but that's a more serious engine issue.
HEADS UP Never yank the starter cord out to its full length and let it snap back—that's the fastest way to kill the recoil spring. Always guide it back in gently.