I'd first check if the ball isn't hitting or bumping something, thus slowing it down.
It's more likely that the plunger or it's bracket is mis-aligned. I'd take apart the plunger assembly and check everything is clean and that moving parts have full and easy travel. The coil sleeve could be worn and dirty and obstructing the punger, or the plunger link assembly may be broken.
I suggest you tinker around. Some patience and persistence will probably solve it.
James.
