When you book from the app it won't work. Using the actual responsive website on a mobile or desktop works fine.
Spending a few minutes here debugging it by the look of it the URL parsed from the app to the booking engine isn't correctly identifying the Flexitime flyer status. The show flexitime flyer option sends a &applydiscountcardpromocode=true to the vbook app but that doesn't seem to actually work.
The simple fix is just to book from the website rather than from the app.