You've anticipated what is the common problem with headset lock washers. The anti rotation tab gets jammed on the threads because the washer gets slightly rotated off it centered position. The first thing I'd try is to grab the washer with a pair of channel lock pliers and rotate it on the steerer. If the tab was off center it will kind of set in place when the tab and groove line up. Then the next thing i do is what you have already done. Try to pry it up with a knife edge driven under the washer. Often you can get the tab to skip over a thread and this then lets you know that the washer needs rotating. Sometimes the tab is taller/deeper then the groove is and is hanging up in the groove. prying the washer up is the only real option here. When off a slight filing down of the tab usually fixes this. BTW when you reassemble it don't rely on the tab to keep the threaded cup from rotating, use two head set wrenches during the tightening. Andy.