Oddly, I solved the problem by taking the hook out and cleaning both pieces really well. There was a sticky, thick brown film between the 2 pieces, like 3-in-1 oil had been used in the past. The film was only between the 2 pieces, not the top or sides of the hook, only in between. Very strange. Everything else was clean, spotless really.
At least the stitches are back to normal. Dang... I suspect 3-in-1 oil was used in other areas as well. It looks like I need to soak her innards in kerosene one of these days.