Searching for reward flight

When i search for return reward flights and the result comes back that there is no availability and i am asked to try alternative dates, a different cabin, etc., why can't the website just display the available dates like it used to? One trick i have learned is to search for reward seat availability as a single journey then the availability calendar does appear for the entire month. The only problem buying two single tickets , say LHR to NYC and NYC to LHR in UCpper Class is that the taxes are GBP 603 versus GBP 432 On an Upper Class return ticket.