Over Stop Microwave Vent Blower doesn't remove...

Question

Magtag Microwave MMV6178AAW is mount for roof vent exhaust. The blower fan is mount for vent exhaust. Blower unit sounds like it is moving air but no air from exhaust port at bottom of unit. Hold a printer paper to the exhaust vents under MV and the paper shows no exhaust is available at the exhaust. The exhaust run in about 30 feet with one 45 degree elbow and 90 degree roof vent, 6 inch pipe. Reported the problem and was contact by technician and he said the it wasn't working, but he couldn't find any thing wrong. Would appreciate and suggestions. Thank you