Have a close look at the handle where the locking pauls come out to engage with the height slots.
The handle is weak in this area and tends to bend if not break all together so the paul will not go down so it will not disengage from the slot.
Every thing that come out of the AYP factory with this mechanism suffers more or less from the same problem and another case of a factory refusing to fix a simple design problem.
On very rare occasion I have seen twigs or other debris blocking the pauls movement but usually the handle is bent .
you can straiten it but it will be weak & just bend again so it will either need a stiffener welded onto it or replacement.
