I believe this can be common with most bike transmissions. I'm not at all very familiar with motorcycle trannys, but from what i'm aware, order for the shift plate or whatever to move through each gear, that gears dog teeth have to line up with the slots on the shift plate. Normally the gears are spinning when you're moving, so that allows them connect easily. If you're stopped with the clutch in, the gears are not spinning, and so if the dog teeth are not aligned with the slots on the shift plate, you will not be able to shift. If you let the clutch out a little bit (or just roll the bike forward or backward), as mentioned by others, it starts turning the gears so the dogs n' slots can align, at which point you're able to shift.
I might be completely wrong, but i think it works something like that. :lol: