Remove the deck so you have a good unobstructed view of the ground drive belt system. Have a helper sit in the seat and push down on the clutch/brake pedal while you examine all the linkages in the mechanism from below the frame.
You are looking for worn or loose pivots, pins or anything that can limit the travel of the components that put enough slack in the ground drive belt for the transmission input pulley and shaft to become "unpowered" so you can shift.
Have you also noticed the clutch engaging at a lower pedal position when you try to get moving from a stop? That can also be a symptom of the same issue.
If you have recently changed the belt and did not use an OEM spec belt there is a chance the belt is a fraction too short.
Basically, something is causing a bit of power to be applied to the belt when the clutch is pushed down.

Here is a link that might be useful: 917.272751 OM

I did replace the drive belt earlier this year and it worked fine for several mowings. With the brake/clutch depressed the drive belt is not moving. I will check for ant worn components.

Applied power does not mean there is enough power to cause things to actually begin turning. Maybe the word "drag" would be a better fit. The belt is dragging against the crankshaft pulley with enough friction that the transmission is TRYING to turn the gears inside, but not enough to actually turn them, because the brake/clutch pedal is also operating the brake, which is why the drag can't actually spin up the transmission.
Another thing to check under the tractor is to make sure that any "belt keepers" that might be in place near the belt pulleys have not gotten bent or out of place which could prevent the belt from gaining enough slack when the clutch is pushed down.

I am buying a replacement belt from Sears. We'll see what that does.

that tractor will shift even if on high rpm and moving a co mmon problem is people do just that they shift on the go and it breaks the internal gears eventualy locking it in nuetral