From your description of the problem I am guessing that you're using one of the 12 (5 kilo) machines.
What's happening is that there's not enough pressure exerted on the rollers to keep them reliably in contact with the bowl surface when the chocolate reaches a certain viscosity.
To some extent your telling us the percentage in your recipes doesn't help because we don't know the fat content of your beans and whether or not you add any cocoa butter to your recipes. You can have two recipes with the same cocoa content with very different viscosities.
Cocoatown is now offering machines that enables users to increase the pressure on the stones to help keep them in contact with the base. This might be what you need. However keep in mind that this extra pressure will probably reduce the life of the nylon bushings, which will cause other problems.
clay - http://www.thechocolatelife.com/clay/