I tracked this down on mine.
It has to do with the weight of the flyback. You can try to track down broken traces or cold solder on this one all day (I did, trust me) and even after resoldering everything and having continuity it would keep happening (I suspect just enough flex to strain a component or trace. Somewhere down the line it seems to be kicking the horizontal amp pot out of circuit which is giving it that half collapse and making it non adjustable.
This might just be an aftermarket fly thing, but if it is original you can give this a go too.
On mine, the weight of the flyback tipped it too far towards the back so it flexed the board a bit. This is even with soldering in the rear anchor. If you push slightly on the flyback to get it to flex (I am talking millimeters here) towards the tube, it should correct the problem.
The stock fly should be screwed into the frame so it shouldn't tip at all, if you have a stock fly screwed in and its doing this, loosen the screw a little and give it a little push and see if it fixes it. If it does, remove the screw and put a small nylon washer (or two between the fly and the frame. Screw it back in. Should sit snug.
To fix this on the aftermarket fly, I used a plastic PCB standoff and screwed it in which put just enough pressure to get it to sit snug and not flex anymore. Pic below. Make sure you put it on this side of the fly because when you screw it in it will push against and not pull out.
Good luck and keep those Polos going.