I have had the same thing happen.  

I tried your formula just now and it works.  Was Sheet 3 renamed?  If not, 3
ideas:

First, are you absolutely sure that column A of sheet 3 is the column with
the data which is being compared?

Second, I would suggest highlighting cells A6 to B1690 in sheet three and
then naming that range (quickest way to do it is after you've highlighted the
section you go to that box right above Column A's header and click on it and
type in a range name--no spaces).  Then replace the $b$6: etc with the range
name in the vlookup formula.

Third--sometimes the formula is working, and the problem is that the names
are really not all equal.  Sometimes formatting does this, sometimes
importing hte data has added blank spaces.  Test the data directly with a
simple logic statement like A6=sheet3!a16.   if all else fails, first rewrite
the formula in the same sheet as the data that you are looking up--put
everytihing in the same sheet right next to each other and see item by item
where the disconnect is happening.--

See http://support.microsoft.com/default.aspx?scid=kb;en-us;315961

I've had similar problems and have solved them by using the ROUND function.
you may need to round off the lookup_value and/or the table_array if either
one of them is a calculated value