Put a shut off valve between the main hydraulic supply and the mast. Raise the mast into second stage, shut off truck and valve. This will identify if it is hydraulic or cylinder. Should mast continue to lower it is in the cylinder. If it does not, check for voltage at the solenoid or through the dash display, diagnostics, hydraulics. Look at the lowering voltage. I suspect a phasing issue at the sequence valve block. It stops lowering once the hoist cylinders lower and does not continue, dig into the valve.