That's odd - can't say I've ever seen that before. I took apart a pusher to have a closer look at that connection between the cap and the post:
Looks to me like a press fit:
So you could press the post back into the cap, but you would have to be very careful not to damage the post in any way - if it get's bent or marred, then it will not likely seal properly. If I was attempting this, I would use a staking set and select a punch to go over the post that would contact the post right near the bottom,. on that angled surface. Then a quick and very light tap should seat it.
If the post can be removed easily, then I would apply some Loctite retaining compound before pressing it back in. But again the surface of that post cannot be damaged, because that's what seals on the O-rings inside the pusher tube.
If all that doesn't work, a new pusher is the next step.
Hope this helps.
Cheers, Al