Is Browse mode access permitted for the field? (check the behavior part of the Data tab in the Inspector).
Is the field from the correct table occurrence? see: Tutorial: What are Table Occurrences?
Is there a transparent object located on top of the field? (Mainly a problem in Web Direct, but if it's a button or web viewer, it can block mouse clicks in Filemaker Pro as well.)
Did you add it as a merge field? (You'll see <<fieldName>> type notation on the layout when you are in layout mode.)
Browse Mode was not selected and that was the problem.