Never done it either but it's a magnetic circuit. There is iron in there whose job it is to "guide" the magnetism from the magnet to the surfaces where you want it, and there is the permanent magnet source.
If you have physical gaps, they won't conduct the magnetism as well as the iron, so slop, wear and tear over time might make a difference to how well it works. For the magnet itself, I'm not sure how you would remagnetise one of these high power magnets.
It looks like it has been tinkered with before you ? Is that a replacement lever ?
Two thoughts. If the bolt was welded onto the shaft they may have overheated the magnet. Magnets die at very low temperatures. Or could it have been reassembled in the wrong alignment ?