DIAGNOSIS Worn or damaged drive links on the chain. Or, a damaged drive sprocket.
FIX Inspect the drive links on the bottom of the chain. Look for hooks, burrs, or excessive wear. Replace the chain. Inspect the drive sprocket teeth for matching wear.
IF THAT DIDN'T WORK The bar groove may be packed with sawdust and chips. Clean it thoroughly with a narrow screwdriver or bar groove cleaner.
HEADS UP Never run a damaged chain. It can derail and whip back. Always match the chain pitch and gauge to your bar.