Diagnosis
This is almost always one of two things: the grease filter is completely clogged (see the other question), or the damper on the outside of your house (where the duct vents out) is stuck closed.
How to Fix It
1. First, clean that grease filter. A dirty filter blocks 90% of the airflow. 2. Set the fan to the highest speed. 3. Go outside and find where the vent duct terminates on your exterior wall. There should be a flapper (the damper) that opens when the fan is on. 4. Have someone turn the fan on high while you watch. The flapper should open freely. If it's stuck shut by paint, dirt, or a bird's nest, that's your problem. Clear the obstruction so it can swing open.
If That Didn't Work
If the filter is clean and the exterior damper opens, the fan blower wheel inside the microwave might be gummed up with grease itself. That requires a more involved disassembly to clean the squirrel cage fan.
Heads Up
Make sure you're using the correct vent mode. This microwave can be set to vent outside or recirculate through charcoal filters. If it's set to 'recirculate' but you have a duct hooked up, performance will be terrible. Check the installation settings.