I can't reproduce that. I did a test:

29 april, Hamburg - Malmö

Normal fares available:
Seat 349
Shared compartment 749
Private compartment 2999

With promotion code INTERRAIL:
Seat 199
Shared compartment 399
Private compartment 1999
 

Did you enter the code in the correct field?

You are right: I must admit that today I tried with other dates - and this time I also had a clear difference! And also different prices than what I had the first time. My trajectory was Hamburg to Malmö.

Looks like they are following the current “modern” habit to offer “random pricing” - meaning: the customer will never know in advance what to expect: “better buy today because tomorrow prices may be double...”

Like the first time, I entered INTERRAIL into the “Promotion Code” field. Entering it into the “Voucher Code” field was immediately refused anyway.

My conclusion is that either there was something not properly working in the online shop the day that I tried first (technical problem), or the “gods of the online shop” sometimes decide that a reduction is today not applicable.

And who are we to know or even understand the reasoning of gods?

Anyway, for me the case is solved now: I decided to stay in a hostel in Denmark. ;-)

(When I was a student, it was possible to travel from Switzerland to Oslo and change the train only once, many really far distances could be done with “through coaches”, and Couchettes were not very expensive. Nowadays most people would fly anyway, and this kind of service has mostly gone. From that perspective, I appreciate the little “renaissance” of night trains anyway!)