Debra Dalgleish has lots of notes on troubleshooting =vlookup():
http://contextures.com/xlFunctions02.html#Trouble

Becksicle wrote:

Hi,

I am working with quite a large amount of data, I have 2 workbooks, they
have one common column - ID, I am pulling information from workbook 1 (48K
records) into workbook 2 (110K records in csv) using a vlookup on the ID
column.

The formula is correct and works for all but about 3,500 records. At first I
thought that these records might not have a corresponding ID, but whilst
doing a couple of manual checks I found that the data was there and just
wasn't coming through. Also the records are grouped together, obviously I
can't check 3,500 records, but I have found over 20 consecutive rows that
aren't coming over into workbook 2, around about the 36,500 mark in book 1.

I have tried re-writing the Vlookup, narrowing the lookup criteria and it
still doesn't work. There seems to be some kind of problem reading the data
in the middle of the workbook.

Has anyone seen anything like this before? Any ideas?

--

Dave Peterson