Gears need to slide by each other in order to perform a shift.
To accomplish this with the most ease, and avoid tooth b;locking, it is best if the gears are rotating.
Since the GEARS in question are down stream of the hydro unit, it is best if the tractor is just moving. rocking will do. It doesn't take much.
With the tractor stationary, and the hydro unit static (input to the gear box stationary), if the gear teeth obstruct each other's passage, there is no possibility of engagement. The gears have some "leed" to them, so forcing the shift lever can produce results, but WHY?
Rock'em Baby!