Reserved Seat Reallocation

I reserved two seats for two passengers to ensure we are both seated together and where we preferred. However, when we were checking in on the morning of the night flight, the seats were not available as they were re-allocated to someone else. The ticket sales at the airport could not assist and we eventually had to be seated in two different middle row seats. Email the airline about the poor service, experience and dissatisfaction with the airline and it's been a week, no feedback, nothing.