Have you changed your return reservation at after it was originally booked, such as an upgrade or date change? That’s often the reason for this event. You need to do one of two things, call BA now and ask them what the problem is. There is probably an amount of money to pay that was not taken earlier, they should be able to process the payment and get the ticket reissued.
The alternative is to arrive at the airport an hour or so earlier than you planned and head to the ticket/service desk. Personally, I’d be making that phone call.