Combo box selection will not calculate
 

I am using 3 combo boxes to select the year, month and day for a scheduling
worksheet. Each combo box is linked to a cell which is 'named' accordingly.
When I select the year and day, the calculations in the worksheet work. But
when I select the month, the calculations will not work.
If I enter the month number into the linked cell using the keyboard and
press 'enter', the worksheet calculates perfectly. Only when the month
number is entered into the linked cell by selecting in with the combo box, it
does not work.
SOMEBODY PLEASE HELP!