Chuck jaws are dirty or worn.
- Open the chuck fully. Clean the jaws and the interior with compressed air or a brush to remove debris.
- Insert the bit fully. Hand-tighten the chuck as firmly as possible.
- Grip the chuck with one hand and give the final tightening turn a short, sharp pull. Do not use a chuck key (it's keyless).
If that didn't work, the chuck mechanism is likely damaged. Inspect the jaws for uneven wear or breakage. A worn chuck cannot grip properly and must be replaced.
Heads up: Forcing a drill with a slipping bit is dangerous. The bit can spin violently or break. Stop use until the chuck is fixed or replaced.