Conditional formatting acting strangely

Hello,

I am using Excel 2003 on XP.

I have a drop down list in column K. Depending on the choices made, I have applied conditional formatting to columns L, M, N, and O (I want them to go grey).

To expand on that:
Choice 1 - the corresponding cell in Column N goes grey
Choice 2 - the corresponding cell in Column N goes grey
Choice 3 - the corresponding cells in Columns L, M, N, O go grey.

When I originally created this in the spreadsheet, everything worked beautifully. The spreadsheet was sent to a user who did not observe any greying out when making his selection from the drop down list. When he sent back the completed s/s, no greying was evident despite the selections made.

When I played around with it, some of the cells tried to go grey but didn't quite get there. BUT, when I changed focus on my monitor to another application then moved back to Excel - voila! grey cells!!!!

Any ideas? Has anyone else found a similar issue? Is there a known problem that I couldn't find when Googling? Is there a way to make it behave? It's useless sending out a s/s to users who then complain they don't know how to complete certain information when I have set up the sheet to let them know to ignore it but they're not seeing the signals.

Any help would be gratefully received.

Anyone?

Please?