Quoted from fnosm:Here are a couple of ideas:
Make sure that the plunger isn't being blocked the auto plunger. The easiest solution for me when this happens is to rotate my plunger a bit and try again.
Right after DI came out there were 'sagging' issues reported and discussed here on Pinside. This may be part of your problem. If you track down those posts you can see how people attempted to resolve it.
One last thing is to see if you have a long or short barrel spring outside of the cabinet on your plunger. If you have a long one you can replace it with a short one(3/4") and that might get you over the hump.
Please let us know what you figure out.
are your playfield hooks bent? Mine were bent and it caused the plunger to hit the top of the ball.