During a recent style set up, I found an issue when trying to label a pressure pipe inside diameter, it was labeling it as ???. It behaved as if it could not find the value.
Checking the used Pipe Label Style it was set up correctly, the issue was not there.
Further investigation, lead me to the Civil 3D objects itself, the Pressure Pipe. If you look at the Pipe Properties, the field for Diameter Inside is empty. This is what caused the label field to show as ???.