I suspect the info is all (mostly) there K, but the problem is that the order it's presented in is so random that it's very difficult to work with.
I don't understand why you're looking for a maximum value rather than a sum, if you're trying to determine what has been shipped.
Your line of code however, should be constructed differently depending on what type of field [JobNumber] is.
You don't say how you have access to the required JobNumber value, so I shall assume, for the sake of clarity, that you have a variable with it stored in. If it is a string value then this will be strJobNo :
- ... = DMax("[QTYBackOrder]","[tblShippingDetails]","[JobNumber]='" & strJobNo & "'")
If it is numeric, then it will be lngJobNo :
- ... = DMax("[QTYBackOrder]","[tblShippingDetails]","[JobNumber]=" & lngJobNo)