CENTERLINES

Select the icon FIRST!

John,

I use centerlines on every project, and have never had any problems with them.  As Ian said, first activate the centerline command, then pick your edges, and I'll bet it works just fine.

If you have a section of a tube, can't you just pick the centerline tool, and click on the round face?  Believe this is how I currently do centerlines.

It seems unlikely that your "display annotations" would be unchecked, since no other annotations would display, but make sure this box is checked.

Without it checked your center lines will not show.