Are you looking for any seats or cheap seats or seat selections?
If you cannot make a reservation at all, that means the flight is sold out or no longer exists. But people can cancel, freeing up seats on a flight previously "sold out".
Airlines don't hold back seats in general, but the number of cheap seats is very limited for certain popular dates.
Airlines do "hold back cheap seats". If you look at a flight combination and find the fare is high, cheap seats could reappear in the future but when cannot be predicted. It depends on how well the airline thinks the seats are selling.
Airlines do hold back seats from the advance seat selection chart. The number varies from one airline to another but typically after 2/3 the seats are selected, the rest of the seat selection chart suddenly goes blank and everybody else has to wait until they get to the airport to select seats.
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