DIAGNOSIS Sawdust and debris are packed into the 1/4" hex collet, preventing the spring and ball detent from gripping the bit properly.
FIX You need to physically dig it out. First, use a small pick, a stiff wire (like a straightened paperclip), or a small flat-head screwdriver to scrape out the big chunks from the collet opening. Then, get a can of compressed air and blast it directly into the chuck from multiple angles. Rotate the anvil (the part that holds the bit) by hand while blowing to clear all sides. Finally, put a drop of light machine oil (like 3-in-1 oil) on a clean bit, insert and remove it a few times to lubricate the ball bearing mechanism.
IF THAT DIDN'T WORK If bits are still slipping or the release collar feels gritty and stuck, the internal spring might be damaged or the collet assembly might need replacement. That's a parts fix.
HEADS UP Never use water or solvent cleaners. They can wash debris further into the gearbox. Doing a quick blow-out with air after particularly dusty jobs is the best preventative maintenance.