There are number of factors to take into account. When you place a seek it has to find someone that also looking for a seek at the same time and that player has to have your rating in their seek range and they have to be in yours. They also have to be looking for the exact same time control settings.
The site also tries not to pair players that have high lag/latency with those that don't. Then, players that are restricted due to fair play policy violations normally get paired with others that are restricted.
I wonder how chess.com says that every day there are about 9000 new members and when you start seeking it takes too long to find an opponent.