autofill district based on postcode input

I need to have the district auto complete based on the postcode entered. I have all of the districts and postcodes in a table that i am using the following to access
=DLookUp(“District”,”tbl_Districts”,”Postcode = Forms!frm_Ch!Postcode”)
This works great for postcode that are 6 characters long, such as S40 3LB.
However, when i enter a longer postcode, such as DE55 4LT, the district textbox doesnt auto complete.
I have verified that these postcodes are in the tbl_Districts.

Any ideas?

Thanks in advance
Lee