There are only a few things that can cause this.
1. Lets start with an easy thing. If anyone has taken a wheel off in order to fix a tire, when they install the wheel again, many times the axle key slips out of the axle and now the wheel spins on the axle and will not let the unit drive.
2. There is a possibility that the pulley on top of the hydro may have sheared allowing the pulley to slip on the shaft.
3. The push-Pull valves (Pressure release valves) may not have completely engaged. Check these again and make sure that they are in the DRIVE position.
4. The hydro's may have cone bad and may need to be replaced.
That is just about all that can cause this other than bad belts or a loose idler spring.
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