What where the cutting conditions??? If it was wet low, where the sickle cuts, then you will have plugging issues. If the grass is real short and FINE it can cause cutting issues if your sickle and guards are worn. Going too fast can cause the sickle to clog as well. Some types of grass are just hard to mow with a sickle cutting system.
So we need a little more information. Wet or dry? Hay thick,thin or short? How fast your trying to go?? What kind of grass/hay?
Usually a NH 488 with a good sharp sickle and square guards will mow pretty well.