1) Bad vacuum pump.
2) Clogged vacuum filter.
3) Clogged vacuum tube or elbow fitting.
Possible Solutions (corresponding to the Causes #):
1) Check for vacuum pump activation (sound). Replace vacuum pump (APR-VPC). Replacement instructions: APR-VPC - Airflow.pdf
2) Replace filter (APR-VF). Reflow Head Inner Locations.pdf
3) Remove and clean vacuum tube and elbow fitting.
If you need instructions on how to replace or check any of these parts, please contact our APR Support Team.