If a John Deere hydrostatic transmission fails to engage, the primary cause is often the bypass valve being left in the "tow" position, which requires pushing the rod inward to restore pressure. Additional troubleshooting steps include inspecting for a failed drive belt, checking for low fluid levels, and verifying that the linkage or keyway hasn't sheared, preventing power transfer to the wheels. For model-specific guidance, consult the John Deere Operator's Manual .