Index Match lookup formula causes #N/A error ... sometimes

[ant...@swbell.net]

Hello,
I have a file A with a list of item numbers in column A and
corresponding quantities in column B. The item numbers in file A were
imported as a text field, the quantity field type is number.
In file B I have the same information, which I typed in by hand.
Therefore, the item number field and the quantity number field types
are numbers.
I have used the following formula to populate file A with the
information from file B:

{=INDEX('[File A.xls]Sheet1'!$D$4:$D$74,MATCH(VALUE('[File
B.xls]Sheet1'!A11),'[File A.xls]Sheet1'!$A$4:$A$74,0))}

The curious thing that is happening is that most of the quantity values
are found and filled into file A properly. However, some of the
quantity values are not returned but the formula rather returns the
#N/A value. I checked to make sure that the item numbers are listed in
both files.

Any idea why this is happening?
All help is greatly appreciated.

Thank you in advance.
Regards,
A. Crawford