Dear Kelley, Yhe first thing I would check is the internal filter. Over a period of time, due to grease and plain old dust, the filter will get clogged. This will prevent the system from operating properly and to it's peak efficiency. Contact the manufacturer for the correct procedures for cleaning and maintenance. To give you an idea of which I speak, fold a paper towel thrice and blow through it. No problem. Next, saturate the paper towel with cooking oil and repeat the procedure. You'll see what I mean.
Len

If the filter is OK, check outside to see if a bird has nested in the exhaust vent, plugging it up.