Thanks for your reply John,
The sensors I am using for testing are Omron Proximity Sensors P/N E2E-CR8C1. I have those coupled to an op-amp to separate the 12v sensor from the 5v Arduino input. There is an LED on the sensor which shows the sensor state, and I also have an LED on each op-amp output. The LEDs are indicating the correct state. I believe it has to do with how the function is declared out in the wiring.h file from what I have read.
Scott