Dear Bret

This is a common Problem in Excel and I know 2 reasons for that:

1) Whenever you integrate Links into formulas (e.g. Sum('C:\[test.xls]Sheet1A1:A10') you need to open the other excel file for an update.

2) You have too many formulas in your excel file.

Sometimes Excel just doesn't calculate although calculation settings are on automatic calculation. The tool below provides you with an extended calculation feature, that fixes that problem:

MTools: Excel Add-In that offers an extended calculation in the case that excel doesn't calculate even in automatic calculation mode

Regards
Marc