Through service and ticketing to Corfe Castle

It was pleasing to see much fanfare about the SWR service to Corfe Castle on Summer Saturdays and promotion here which seems a good idea and a strong show of cooperation between SWR and the Swanage Railway

however I cannot seem to buy this £10 promo ticket online from Salisbury for any other date besides Saturday 9th June even though there are no apparent engineering works affecting the service I can see. Is there a way to improve the information & ticketing of this service and when it is likely to run this summer?