Excel 2007

The font of cells in column A is set by Conditional Formatting to turn
gray if the same-row cell in column B contains data (i.e., <""). The
conditional formatting works if I type data into the cell in column
B. However, if I paste data into the cell in column B, Conditional
Formatting is not automatically triggered. I have to press F9
(calculate) to invoke Conditional Formatting. Similarly, if I delete
the data from the cell in column B, the Conditional Formatting doesn't
go away until I hit F9. Is there some option I should check or
uncheck that will make Conditional Formatting work automatically in
these situations?

Thanks.