In general I prefer poly monofilament to nylon. You don't run the thread through a hole in the net. You may have to move the longarm slower than usual, especially if going right to left. Check your thread (by trying to snap it with your hands) to see how strong it is; nylon thread sometimes becomes brittle and breaks easily, and if that's the case you won't be able to use it. Also check the thread path, including the needle, for any burrs or hang-ups. I'm assuming that the breakage is on top - are you also using it in the bobbin?