If you're feeding grain into your 2 roller mill via a funnel, make sure that the spout of the funnel is not too close to the rollers. It has to be high enough so the grain can distribute itself over the entire length of the roller gap. I had this problem and almost never figured out what was causing it.
Also, especially when you're first getting started, you might need to give the non-driven roller a little nudge.With the motor running and being very careful, just take your fingers, lay them on the outside of that roller and pull up on it, IE start it spinning it in the correct direction. It doesn't take much force and that will usually get it going. It may stop again, but if so, just repeat this step, just get those fingers out of the way once it gets going.
I can mill 25# of grain and never have it stop and other times it stops 3-4 times during that same size batch. IMO it has to do with the moisture in the grain, but that's just a guess on my part.