The most common failure of business card slitters, that the spindle (1) curves and the movable knife (3) jumps to the other side of the fixed knife (2). This is happens usually because a too heavy cardboard sheet (6) or if there are more sheets forwarded into the machine.
The solution of this problem is not so easy: If you reposition a knife to the original position the springs (4) pushing the other knives out of the proper position.
I show a possible solution for this problem on a Cyklos visit card slitter. (The other visit card slitters are usually similar.)
If you disasamble the knife block you can remove the movable knives to the desired position.
You can fix the movable knives in this position with a piece of nut or bolt as in the photo.
Assamble the knife block, remove the nuts and you can use your favourite visit card slitter with great joy.