Monofilament stretches, so you have to lower your upper tension when using it.

I have used YLI with no problems, but I use a cover on the spool and feed it through an upright cone spool thingie (what others have described). I have also used it with an "outrigger" (just a rod that slips over the regular spool holder with a hole for the thread to feed through).

Are you using it in the bobbin too? If so, you have to be careful of the stretch when winding the bobbin. Wind slowly, to minimize stretching. Especially if you use plastic bobbins, fill the bobbin only half full. This is because monofilament can distort a plastic bobbin towards the outer edges. If using a metal bobbin, you need to leave only a little space at the outer edge.