If you are familiar with the VLOOKUP command in Excel you may be interested to know that Vlookup counts hidden columns as well as the visible columns when deciding which columns information to bring back. So perhaps your VLOOKUP is bringing back the wrong column? This could be the reason.
NEW: Go to VLOOKUP Explanations with downloadable exercises and detailed solution
In the VLOOKUP syntax of
VLOOKUP(lookup value, table array,col index number, [range_lookup])
the Col Index Number portion specifies which column of the highlighted table array should be pulled through. It is important to note that when Excel counts the number of columns in the table array it INCLUDES any hidden columns.
So if you cannot understand why the column number used it different from what you counted, it may be that there are some hidden columns.