DIAGNOSIS The self-tightening jaws inside the chuck are jammed with debris (concrete dust, metal shavings) or the internal mechanism is worn from not being hand-tightened properly.
FIX This is a cleaning job. You'll need a can of compressed air or an air compressor. 1) Open the chuck jaws as wide as they go. 2) Blow air directly into the jaws, angling it to get behind them. Rotate the chuck and repeat. 3) After blowing it out, put a few drops of light machine oil (like 3-in-1) on the chuck threads—the part that moves when you turn the sleeve. 4) Work the chuck open and closed by hand a dozen times to work the oil in and loosen any grit.
IF THAT DIDN'T WORK If it's still slipping, the jaws themselves might be worn smooth. For the 2904-20, the chuck is a replaceable part. You can order a new Rohm or Milwaukee chuck. Replacing it requires locking the spindle with an Allen key (usually a 10mm) and using a pipe wrench or chuck key to unscrew the old one—it's reverse threaded.
HEADS UP Always hand-tighten the chuck firmly after using the tool's power to snug it. Relying solely on the drill's burst to tighten puts all the stress on the internal gears and wears the chuck faster. A good clean and lube every few months if you're in dirty environments makes a huge difference.