Diagnosis
The dust port or the internal shroud is blocked. Fine dust creates a hard plug.
How to Fix It
Detach the vacuum hose. Use a small, stiff brush (a dry paintbrush works) to scrape out the dust port. Then, remove the sanding pad. Look inside the shroud around the spindle. Use compressed air to blow out the internal channels from the inside out. Do this outdoors.
If That Didn't Work
Check your vacuum hose and shop vac filter. If they are restricted, they create back pressure that clogs the tool's port first. Ensure your vacuum has strong suction.
Heads Up
Never use a wire brush or metal pick to clean the plastic dust port. You will scratch it and make future clogs worse.