Night Riviera were unable to provide us our berth. Is this common?

I am just waking up after a very rough night on the GWR Paddington -> Penzance

Two of us had booked a sleeper berth but when we arrived, we were told the carriage in question was out of use. Our only options were take a seat (with a refund) or travel the next day.

How common is this? It seems totally unviable if carriages are regularly put dark at the last minute.

Also, why is the seated option this poor? Bright lights through the whole carriage, hard seats with solid armrests. Very difficult to sleep even when passengers are behaving