Diagnosis
Most common cause is a worn or damaged hammer/anvil mechanism. The kinetic impact parts are fatigued.
How to Fix It
Inspect the anvil for chipping or excessive wear. Listen for a 'clacking' sound instead of a solid 'bang' when running free. This confirms internal wear. The hammer case and anvil need replacement as a set.
If That Didn't Work
Test with a known-good, fully charged 5.0Ah or larger battery. A weak or old battery cannot deliver the required peak current.
Heads Up
Do not use penetrating oil inside the hammer mechanism. It washes out the factory grease and causes rapid wear.