Headers won't update in 2007 Excel workbook

[Bean Dinner]

I have a workbook created w Excel 2007, has about 50 sheets, half of which are charts based on the data in the other sheets. Every time I update the data, I need to change the sheet headers. The headers are all the same: "Quarterly Monitor Through month, day" All I need to do is change that month & day. It doesn't seem to matter whether I group the sheets and try to change them all at once, or do them individually, I always have to go through the process more than once. I double-click in the header area, select the text I want to change, type in the new text, and it looks fine. Then when I click in the grid area again, the change isn't there. It reverts back to the previous text. Usually if I do it again, it works, but not always. I generally have to go through every sheet individually to make sure the header is correct. Sometimes it looks correct, but when I print it out, the old header shows up again. This seems bizarre to me. Any suggestions???