There are many reasons for a stove to not drop pellets. Use these steps to help diagnose the issue.
The hopper should have enough pellets in it to create vacuum. It does not need to be full but should be at least a large bowls worth of pellets.
The hopper and door sensors should read in a closed position. Newer model stoves will not drop pellets if the hopper lid or the glass door is open.
All stoves except for Mt Vernon AE, E2 and Trekker have a call light. A red light should come on when the thermostat calls for heat. This light is usually near the power cord coming off the stove. Be sure look behind the panels on the side. AE models should show "Startup" on the thermostat. If this is not the case, the issue could be the thermostat or thermostat wire.
Use the supplied clean out tool or a bent coat hanger to check the drop tube for an auger jam.
Ensure the stove is clean. You can tell if it is clean by reading the Dirty Door Glass. A dirty stove can prevent the stove from dropping pellets.
Remove the pellets from the hopper looking for foreign objects. Vacuum out remaining pellets that cannot be scooped out. When starting the stove again you will need to hit the reset button or hit "Retry" (for a Mt Vernon AE). Do this until pellets drop. If pellets fall but do not light. You will need to try one more time.
If the problem is not yet solved, a new feed motor could be needed or possibly a new control board/box.