As you mentioned, your concaves are likely worn badly enough that they won't retard the flow of material properly. If you're finding straw lying between the concave crossbars, the rotor bar teeth are probably losing their leading edge and teeth are rounding over so they won't carry MOG. To try to help you through, I'd suggest that you rezero your concaves. likely, they're presently set tightest on the bottom and widen out as the rotor turns toward the R_H side (from the seat looking forward). Try to rezero your concaves to where the tightest point is about 1_8" away from the rotor(or maybe a little tighter) at about the seven o'clock position looking back from the front, and set the bottom at about 1_4" away from the rotor. What this does is to provide a more constant wedging action against the crop even when you open the concaves wider for feeding, and extends the area in which the crop is retarded. Only drawback you might see, but not likely, is a little more loading on the R_H side of the shoe. Also, operate the rotor as slowly as possible for your particular ground speed in order to load the threshing area (crop on MOG thresh). Hope this is all understandable.