In my experience its down to a bad image convert, there is "muck" left instead of a clean line so the blade is actually cutting this also.
Im not sure on your skill level so sorry if you've already checked and ticked this off only this happened to me all the time when i first started out and it took me a while to figure out, try zooming right in on the area of the design that gives you issues on the computer and just check there are not pathways other than what you expect.
Im not sure on your skill level so sorry if you've already checked and ticked this off only this happened to me all the time when i first started out and it took me a while to figure out, try zooming right in on the area of the design that gives you issues on the computer and just check there are not pathways other than what you expect.