Take a look at the photo. Is the center portion worn or chipped? It's so smooth that it almost looks like it was made this way.
It is worn!
That was my first thought. I'll order a new one. I want this one to operate reliably for a very long time.
That yoke is badly worn. I would replace both the yoke and the plunger. We make our plungers with a full width nose so they do not wear as quickly.