Troubleshooting hand sealers
Try holding down the
seal bar a few seconds
AFTER the "Click" of the
Micro switch to allow the
seal to cool.
For a more professional
looking seal
Also see Maintenance Tips Replacing PTFE Covers and Heat wires
Hand Impulse heat sealers wholesale
As you stated it sounds as though the customer has checked already to see that the Micro Switch is
activated each & every time the seal arm is brought down to make a seal on the bag.  They should be
Also, check to make sure that there are no loose wires that would cause the unit to work intermittently.  
Check the plug going into the timer & other wires too!

Next check to see if there is play in the arm of the sealer.  If there is play in the arm it will wiggle causing
the arm when brought down to not come down evenly but cockeyed.  This will cause the sealer to act as
if it is not working.  To fix: Lift the seal arm up & where the arm attaches to the base at the hinge you
will see a screw or set screw that will need to be tightened so that the hinge pin is kept snug in place.