Double selling exit row seats

I am a club member of BA and have flown to Cape Town 8 times with BA Comair since 7 March 2019 and have paid and booked every time for exit row seats ( R 168,00 each time) four times I was allocated other seats so BA took R 168.00 x 4 = R 672.00 of my money. When I ask them why am I not getting the seat I paid for the answer is that the planes have been changed. On every flight the exit row seats were full so they have been double selling the seats at least once! The contact centre promised refunds since 8 March 2019. I followed up by phone and mail but Mr David Holland doesn't return my calls.