You might try placing a new copy of the field on your layout and applying the pop-up calendar to it. You might even try a completely new layout for this test. (These tests check to see if you might have some layout corruption going on.)
You can also check this link, for a possible explanation as you indicate that you are using Mac OS 10.6:
Thanks for that suggestion. As it so happens, just a few hours after I had posted my query the problem resolved itself and all of a sudden the drop-down calendar dropped down obediently and right up to the time of writing it continues to do so. Before that I had tried making a clone of the layout and then reimporting the records and it would work the first time and then fail thereafter. As it is, all continues to work so I shall not try to 'fix' anything for now. Perhaps it is possible that I did something amiss in operating the calendar but I can't think what that might be. In any case I hope I won't need to have recourse to any solution in the future, but I shall save the suggestion you made with many thanks to you.
I have been able to reproduce the problem! If I drag the FileMaker window to a secondary monitor the drop-down menus cease to work. It would also seem that merely dragging it back to the main window alone fails to correct this; the application must be quit and relaunched.....
I would be glad to be able to use the database from the second monitor, but I'll live with having the calendars work for easier data input... However any solutions would be welcome...