rear windshield wiper not working properly

When I use the switch on the dash to turn on the wiper blade to clean the back windshield, all I hear is a thump, and as I look in the rear view mirror I don't see the blade wiping the glass. I tried the low and high on the switch and all I hear is the "thump" sound. should I change the switch first and then maybe the interior motor switch inside the back lift gate panel? It seems as if the arm of the blade doesn't want to come up to sweep the windshield. any help on this issue.