Good Day Richard
Problem: Not able to filter a field in a query resulting from a DLOOKUP expression.
Just as you predicted, I m having trouble getting DLOOKUP to work satisfactorily.  A Payments table records member payment including those for subscriptions to a magazine (Wings); fields in this table relevant to the problem are; [ReceiptDate] (a date field) which records payment date, and [WingsComplementary?] which is true for members who receive the magazine without charge.  Another table named WingsEditionLookup provides an array with [ClosingDate] related to the publication date [EditionDate].  Again these fields are in date format.  They have been sorted Oldest to Newest.
The expression in the query is: FirstDelivery: IIf([WingsComplimentary?]=True,Null,DLookUp("[EditionDate]","WingsEditionLookup","[ClosingDate]>#" & [ReceiptDate] & "#"))
The IF function overcomes the error produced in the record for those members who qualify for complimentary copies of the magazine (because the [ReceiptDate] field is blank).
The query runs with this expression.  But there are two problems.  The [EditionDate] field returned by DLOOKUP is not always correct.  The other problem is that I m unable to filter the field [FirstDelivery].  Access generates the following error message: Syntax error in date in query expression  [ClosingDate]>#
In an attempt to trouble shoot the problem, I changed the date format in the relevant tables to Number and modified the DLOOKUP criteria to  [ClosingDate]>  & [ReceiptDate].  This overcame the problem of incorrect look up of [EditionDate].  But, the filtering problem remained.
This time the error message was: Syntax error (missing operator) in query expression  [ClosingDate]>
Sorry for being so verbose, but it is the only way I can describe the problem.
Best Regards Richard.