I've been having the same problem with my crankenstein 2 roller mill of the slave roller not engaging or losing its engagement. Like others mentioned, a quick flick of the slave roller will get it crushing again but it's a PIA. I've had this motorized for several years now and used to love it as I could leave the 25+ lbs of grain to mill and get the rest of my gear set up. now I have to keep an eye on it to make sure it's still grinding.
I'm now looking at either a 3 roller crankenstein/monster model or the jsp maltmill, which is the only one that I've found that has an option for a gear driven second roller. Anybody have experience with this gear drive option?