2003 Cadillac Deville Oil Pressure Sending Unit

If you have the sensor rotated to where the lock tab is interfereing with the mount bracket, that may be causing you some grief, rotate it to where the tab can have clearance. It is possible that you may have bent a pin over in the sensor, and that could be denying you as well. That could be difficult to see, may require a flashlight and a mirror. You can also undo the 10mm bolts that hold the oil filter adapter to the block. If you loosen them, the adapter will have a slight bit of play. You can remove them and lower the adapter as well. I often replace the sensor with the adapter out of the car, to me it is easier.