Bent shifter forks are a common problem preventing complete shifting. Also if it has come out of gear under power it will round the dogs and cause tapering which will cause it to thrust back out under load. Need to see if the shifter shifts it into a position completely and if so, is there anything but uniformly flat contact surfaces between the two. If both of these check out look for a problem with the shifter barrow or detent or one of the shafts and their bearings. Double check all your thrusts are in the correct place as well.