On Monday my wife wanted to use the Patrol. She started it, and moved the gear leaver into Reverse. She stepped on the accelerator, but nothing happened. The Patrollie did not move. She then shifted to Drive and stepped on the accelerator. Still the Patrollie did not move. Fortunately so, because the garage wall was in front of it.
She went through this sequence 3 times, but the Patrollie refused to move. She left it at that.
Yesterday morning I tried it. Same thing.
I played around with the gear lever, by shifting it from Park to Drive to Neutral and back.
When I shifted to Reverse again, it worked. No problem.
I have used the Patrollie yesterday and today, and everything works fine.
Could anybody explain this?

(I did wash the engine bay on Saturday, with a hosepipe, not a highpressure hose. But it worked fine on Sunday).
I would really appreciate any suggestions.