Most meters have a diode setting, but reading noting one way and a resistance reading the other way sure sounds like the diode is good.
The fact that they're vibrating suggests what others have stated, either the barrels are a bit on the slippery side or you have too much vibration, or both. If you have them really bouncing around, they don't go up the ramp. Mine works best around 11-12 volts, they run right up the ramp.
I tried it from 8 -18 volts and anything below like 14 they barely move at all. If I put the barrels on the ramp manually they actual look like they are trying to go the opposite direction. I made sure its level and I tried screwing it down to platform at different tightness's. Adjusted the coil a million times but there only so many gaps you can adjust it to. I'm baffled. This is such a simple design. The only thing left is the coil itself. Not sure what else to try. Hate to throw away a brand new barrel loader but right now its as useful as a brick! haha
