There is a metal bar with a small tab that fits into a small notch at the top of the hook assembly. There is a small slot immediately behind the notch where the needle enters. The tab keeps the slot from rotating with the hook. If the slot has moved, when the needle comes down, it will hit metal and not enter the hook area. That could cause your needle breaks.
There will be a tiny movement because the tab is slightly smaller than the notch, but the notch should not be able to move past the tab.
There has to be a gap between the tab and the back of the notch of about the diameter of a needle. Too little and the thread can't pass between smoothly, too much and the notch will rotate out of position.
It may be easier to see if you remove the bobbin, needle plate and side covers. Rotate the main shaft by hand watching as the needle enters the hook area. Look at the top of the hook assembly as the needle penetrates and you'll see the slot that I mentioned. Adjust the retainer tab by loosening the two screws. The centre of the tab should line up left and right with the centre of the needle as it penetrates. I use an old needle between the tab and notch to check the front to back clearance.
