Sounds like you need to rebuild the optimized table. To do this just add the VERIFY tag to your query like this:

SELECT * FROM Customer VERIFY WHERE IsActive = 1 and FirstName = 'FirstName' and LastName = 'LastName'

See: How do I setup the QODBC Optimizer? Where are the Optimizer options? for more information.
See also: MS SQL Server 2000 Linked Server issue using the QODBC optimizer 

Just use:

SELECT * FROM Customer CALLDIRECT WHERE IsActive = 1 and FirstName = 'FirstName' and LastName = 'LastName'

instead.You can also override your QODBC Optimizer configuration screen settings to suit what you are actually doing by using the following tags:

VERIFIED | VERIFY - Forces Full Resync with QuickBooks on the optimized table before Query starts
CALLDIRECT | UNOPTIMIZED - Passthru query directly to QuickBooks - use no optimizations
OPTIMIZE | OPTIMIZED | NOSYNC - Passthru query to optimized table directly for maximum speed

Examples:

select * from InvoiceLine UNOPTIMIZED
select * from InvoiceLine NOSYNC
select * from InvoiceLine VERIFIED

For example, to read new (recent) InvoiceLines directly out of QuickBooks use:

select * from InvoiceLine UNOPTIMIZED WHERE Txndate > {d '2008-04-01'}