I have noticed something, when the frame/blades are lowered directly down onto the paper guides. Because they are spring-loaded they tend to keep the blades elevated a mm or two above the paper, the blades won't actually make contact with the paper. Specifically, this is the upper AND left-sided blades because that's where the paper guides are - the lower and right-side blades do (I imagine) make contact with the paper.
Not having the chance to try for myself until I get the spares, I was wondering if that means the black neg-edge borders I try to do on those two offending sides might not be sharp.... ???