Linking to file, some formulas work only if file is open
 

I am in Excel ’07 and am linking to another file also in ’07. I have some v-lookup and sumif formulas on my data tab. The vlookup formulas work regardless of whether the file I am linking to is open or not. However, the sumif formulas only work if the file is open. Both are referencing the same tab of data. Not sure what is going on. Any ideas why the sumif will only work if the file is open? Thanks.