I had a situation a week or so ago where ba.com would not let me book a fare which it was offering. It was an L class outbound with a B class inbound, all offered as normal, but then it would not price it or let me proceed.

So I booked a higher fare, only to find that the next day the same lower fare was offered, but this time it did price and could be bought. I refunded the higher fare under the 24 hour rule and bought the lower fare.. very frustrating..