Les,
It would be helpful if you also posted your actual formula...
My guess is that your data range is not sorted and either you didn't pass a
4th parameter to the VLOOKUP or you did pass a 4th parameter but it was set
to TRUE. Try setting the 4th parameter to FALSE...
From Excel Help
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VLOOKUP(lookup_value,table_array,col_index_num,range_lookup)
If range_lookup is TRUE, the values in the first column of table_array must
be placed in ascending order: ..., -2, -1, 0, 1, 2, ..., A-Z, FALSE, TRUE;
otherwise VLOOKUP may not give the correct value. If range_lookup is FALSE,
table_array does not need to be sorted.
If VLOOKUP can't find lookup_value, and range_lookup is TRUE, it uses the
largest value that is less than or equal to lookup_value.
If lookup_value is smaller than the smallest value in the first column of
table_array, VLOOKUP returns the #N/A error value.
If VLOOKUP can't find lookup_value, and range_lookup is FALSE, VLOOKUP
returns the #N/A value.