I have a weird issue in Excel 2007

If I have two workbooks open. I choose the cell I want data to go to in
workboo1 so I put in "=" (without the quotes) in that cell. Then I go to
workbook2 and choose a cell that has data in it. I hit enter and nothing
happens. Usually it sort of highlights around the cell and when I hit enter
the data from workbook2 goes to the cell in workboob1. But that isn't
happening.

I had someone with the exact same issue, and they just kept trying it over
and over and it worked one time, then it stopped working again. What am i
missing?