It sounds like your level surface is not very level. Precision levels are not linear when you have themwaty out of level so although you making changes it doesen't show up in that orientation. When you flip the level and adjust, you are seeing a change. You need to keep going.
You can a level position on your surface by rotating the level and observing the bubble. You are seeing a difference when you flip the level. What you should see at some point is the left side bubble creeping more off center but at some orientation the right side bubble will start to move towards the center. Ideally, you would want the deflection to be equal but if the level is far out of calibration, that may be difficult to notice. Nevertheless, this would be a good place to sart the calibration process. You should see both sides start to come toward the center. The level is calibrated when the offset is equal and on the same side of center. The surface is then level when the bubble is centered.
Niether the bullseye level nor the digital level are sensitive enough to insure that you are in the ballpark with your"level" surface with your precision level. I have both. I would prefer a good mason's level and I would examine the bubble very carefully, flipping the level and looking for exactly the same position for the bubble.
