It's shifting thru the gears with you helping it, so the problem isn't inside the gearbox.
most likely it's the shift shaft flange thingy (I forget the word) not returning to it's nuetral position after shifting, thats why you helping by manually pushing it down with your foot helps.
time to pull the clutch cover and inspect where the shift shaft rotates the shift star, there's 2 spring ends that must be hooked inside the flange thingy, that return it to it's nuetral position after shifting.
helps to have the clutch/basket out of the way to see whats going on down there between shift shaft and shift star.
you need to be rotating the engine output gear to get it to shift while you inspect, either with chain off/by hand or rearend on jack spinning the wheels.
I've also heard of too long of magnetic oil drain bolt causing similar symptoms