VLOOKUP data format problem

I frequently have a situation where data I'm searching for exists in the search range, and is formatted the same as the search criteria, yet returns a #N/A. If I select the cell in the search range and hit F2 and enter, the lookup then works. First, what causes this, and secondly, how can I make the lookup work without having to hit F2 and enter for every cell in the search range?

Go to Excel Options -> Formulas and check is it set to Automatic Workbook Calculation. If not change it.

I failed to update my profile to show Excel 2010. My calculation options are set to automatic. Is this the same as Options - Formulas - Automatic Workbook Calculation is previous versions?