What would cause low suction?
Has the customer noticed any of the warning lights flashing, warning that the filters or motors need service?
Is there a blockage in the hose between the mill and tour vacuum unit? Remove the hose at both ends and shake it out.
Is the large filter cartridge consumed ( does the shaker motor work?), clean out the filter.
When unit is on and working, perform a manual shake of the shaker to check that it is working. (push and hold the power button until there is a long beep, release button, unit should stop, do a shake and then resume working)
Is the Hepa filter consumed and not allowing the air to pass through? Is there debris in the upper filter section? If there is, the Hepa is most likely consumed.
Remove back motor access panel ( 6 screws), and look to see if both the motors are working, or if one is not. This could show that the motor brushes are finished and the motors will need replacing. ( see attached PDF to see if the brushes are consumed).
Filters are: F082 cartridge, and F191S Hepa.
Motors are AR175 and they will require 2 of them. (It is always recommended to replace both motors at the same time, as they are both the same age)