Can you share the SQL that the query produces?
I dont tend to use LOOKUPS, there might be a issue of instead of it being a number its a string so youd need to wrap the value in single quotes.

Arthur emailed me his SQL. It looks like this:

SELECT Customer_tbl.BusinessName, Enquiry_tbl.Site, Enquiry_tbl.EnquiryDate, Enquiry_tbl.EnquiryNumber, Quote_tbl.QuoteNumber, Quote_tbl.QuoteDate, Quote_tbl.QuoteConclusion, [Quotedate] 30 AS QuoteExpiryDate, Date()-[QuoteDate] AS DaysFromQuote
FROM ((Enquiry_tbl INNER JOIN Quote_tbl ON Enquiry_tbl.EnqID = Quote_tbl.EnquiryNumber) INNER JOIN Customer_tbl ON Enquiry_tbl.CustID = Customer_tbl.CustID) INNER JOIN CompanySiteName_tbl ON Enquiry_tbl.Site = CompanySiteName_tbl.SiteID
WHERE (((Quote_tbl.QuoteConclusion)="Pending"))
ORDER BY Customer_tbl.BusinessName, Date()-[QuoteDate] DESC;

I wouldn't put any kind of calculations in your ORDER BY, like Date()-QuoteDate. Try getting rid of that. I'd start there.