If you have an iPhone or iPod, you can download a free app called iLevel. It uses the accelerometer inside the device to turn it into a digital level. Be sure to follow the instructions to calibrate it on a known dead flat surface.
I would be surprised if a similar app was not available for non-Apple handheld devices, if you don't have an iPhone.
Take off the glass and use the level directly on the surface of the playfield. Take measurements at different spots.
Get the side-to-side at dead zero level.
Get the upward slope somewhere between 6.5 and 7.0 degrees, generally.
Most games have a sweet spot to get them dialed in.
Are you using fresh shiny smooth new pinballs?
Is everything nice and clean? Playfield waxed?
Where and at what time during play is the ball sticking?
Please be very specific.
RussMyers