Can you post your formula here with a bit more description?

If you are using VLOOKUP with the fourth parameter set to TRUE or 1 or
omitted, then the lookup data will have to be sorted and your result
will be the value in the lookup table closest to (and less than if not
an exact match) the search data. However, if this parameter is set to
FALSE or 0, then you will be looking for exact matches and the data
does not need to be in any particular order.

Hope this helps.

Pete