DIAGNOSIS Debris is jammed in the collet, preventing the ball bearings from gripping the bit shank properly. This is a maintenance issue, not a breakdown.
FIX You need to access the collet. Pull the chuck sleeve forward. Use compressed air to blast debris out of the exposed jaws. For caked-on grime, use a small brush (like a dry paintbrush) and then the compressed air.
IF THAT DIDN'T WORK The ball bearings or spring inside the collet might be gummed up. Apply a single drop of light machine oil (like 3-in-1) into the collet, work the sleeve back and forth, then wipe away excess oil and debris.
HEADS UP Never use grease or heavy oil. It will attract more debris. If cleaning doesn't restore a solid grip, the collet assembly is a replaceable part—it's cheaper than a new tool.