Queries :: DLookup Not Working With Certain Dates

Can't figure out why dlookup will not work for me in a query for certain dates?

Simple example to highlight the problem:

Table1 -

Read_Date----Read_Value
31/07/14-------10
01/08/14-------20
03/08/14-------30
20/08/14-------40

Query based on this table with the following calculated field:

Expr1: DLookUp("[Read_value]","Table1","Read_Date=#" &[Read_Date]& "#")

Query output -

Read_Date-----Expr1
31/07/14--------10
01/08/14
03/08/14
20/08/14--------40

Will not return a value for 01/08/14 or 03/08/14 (dd/mm/yy)

If you try it with many dates its skips several and I cannot see a pattern.