Remove your needle plate to be able to fully inspect the knives. Under the needle plate you will find a silver articulating knife and a black knife that has SWF engraved on it. Look at the leading edge of the black knife and inspect it for any burrs or nics. Use your thumb nail to detect if it has a nic on it. If it does then it will have to be replaced.
If you swing the silver knife out from under the black piece and loop a piece of thread around the front of it, push it slowly under the black piece all the way and see if the thread trims, If it does not then the angle of the black o piece is not set correctly and will need to be adjusted. It would be like the screw coming loose on a pair of scissors would not allow you to cut paper, the same lies true with that black piece. If the angle or height has changed then it will not cut at all.
For both of these scenarios I would highly recommend either you send the knife block in for adjustment or if there is a nic in the black piece then I would also suggest you send the knife block in for repair, as setting the angle and height of that black knife has to be set exact. A trained technician could do this in about 20 minutes and without ever seeing one you can very well be at it for days. Setting the angle and height and then remounting it to the machine put the needle plate on and test the trim. If it fails then you got to remove it again and try a different setting. You can see how this would build to a frustrating matter real quick.