Bad service

I gave my tab in for repairs while under warrenty, 2 software updates has been done. When i got the tab back i immidiatly booked it in again. 14 days later and i phone to find out what is happening and has been told that the motherboard is faulty and needs to be replaced, but it will cost me R2800 because the tab is now out of warranty. How can it be that it wasn't picked up that the motherboard was faulty while it was under warranty? Sibu could not answer me and would not let me speak to a manager. She just kept on telling me the handset is out of warranty?