So I just built my first 3D printer, a Kossel 2020 from Folger Tech. I used Marlin firmware. It seems to work. The bed and hot end heat up to temperature, it moves and when I print something it seems to print it in the right place. However I can't get the extruder motor to do anything! I click extrude on Pronterface and try to feed the plastic through but nothing happens. The only thing I can think that has happened is the A4988 for the extruder burned out. I can't find anything else online though.