See if anything is blocking the drive plate assembly from moving into the reverse position when shifted into reverse with the shifter. The shaft that the drive plate moves on could be rusted or something not allowing it to move into the reverse position? The drive plate assembly is the flat round disk driven by a belt from the engine crankshaft and moves back and forth with the shifter under the mower. The RM830 series or model series 927XXXX was a great mower as was the 925XXXX series before it.