The fact that you are almost going thru the wax paper backing at higher force, is too much blade exposed.  It is not possible if the blade depth is set correctly Oracal 651 should never have to use 2 passes.  It is only 2.5 mil thick. With that Clean Cut blade you may also have broken the tip off, very easy to do. It takes much less force than regular blades. This is the correct way to set the blade depth.  To start with, you should set your blade depth correctly, by taking the blade holder out of the machine, and firmly cut across a piece of scrap vinyl, you will be cutting. You should only be cutting the vinyl and barely a mark on wax paper backing, Adjust blade to get there, Then put the blade holder back in machine, and use the force of the machine to get there, same results, only cutting the vinyl and barely a mark in wax paper backing. You should barely see and feel the blade out of the blade holder. Regular sign vinyl is only 2-3 mil thick. You only cut with the very tip of the blade.   
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