Why does your riding mower leave an uncut strip down the middle?

probably center blade on deck is jambed, or blade is broken and or missing

I am having the same problem with may 42" Scott's riding mower. The blades are new (there are only 2 blades , no center blade) and are installed properly. The belt is also new and installed properly. The uncut middle area is more predominant in thicker or taller grass. In the center of the front of the mowing deck is a roller (to keep the deck from bottoming out ) which may be causing the grass to flatten and not be cut. If anyone else has had this problem and has a solution I would be interested in it.