DIAGNOSIS The dust extraction seal inside the chuck is missing, damaged, or clogged. It's a small rubber ring that fits around the bit shank.
FIX Remove the SDS-plus chuck. Inside the chuck barrel, you'll see a groove near the front. The seal (Bosch part number 2 607 001 041) sits there. Clean the groove of packed dust. If the seal is torn or missing, replace it. Ensure it's seated evenly.
IF THAT DIDN'T WORK The vacuum adapter or hose might have a leak. Check the O-ring on the tool's vacuum port and the connection points on your hose.
HEADS UP Never operate the hammer without the dust extraction seal if you're using vacuum attachment. It drastically reduces vacuum efficiency and exposes you to silica dust.