The date for pricing condition is taken either from the pricing date on the header or on item level -> pricing date. 
Probably you use currently the pricing date as order creation date. In some business scenarios it is more appropriate to use requested delivery date for pre-defaulting the pricing date. In your case you wish to have the pricing date between these two dates, which does not seem a very logical business concept.
Unless you explicitly tell the system to use a different pricing date (that is in the validity range), the discount cannot be determined automatically.
Based on what criteria the system should pick 20.09.2015 for pricing purposes- do you have any business rules for building the logic?