That is probably the deficiencies in BA's website. If you did not yet try searching by multi city option specifying your schedule, that may work better.

Otherwise you'll have to phone up (or go to an airport), you may be able to get the agent to waive the phone booking fee if you say the itinerary didn't show up on the website. It is not guaranteed but some agents will waive the fee.

BA's internal systems are fully capable of ticketing across their JBV (and more) with any flight codes but their website is not a high quality piece of software.