It could be a 'married sector' issue, where you cannot book reward seat A without B connecting.

You don't see this much on BA, but it is more common on other airlines. I had it with Lufthansa last year when trying to book something.

There is usually some logic to it. If, for eg, there was heavy demand for point to point Manchester to London redemptions, BA may decide to hold back some reward seats on each MAN to LHR flight for people booking a long haul connection, to give people in the regions a fair shot at bookinga long-haul plus connection.

It is more common on revenue tickets. It would be commercially crazy for BA, for example, to sell every seat MAN to LHR for a domestic if it meant that someone wanting to pay £4k for MAN LHR CPT in CW couldn't book it because there were no domestic seats.