The quiet player may be enjoying the group more than the competition.

Not everyone at the table is playing the same game. One person is there for the contest, working out the best move; another is mostly there for the company and would be as happy doing a jigsaw with the same people. Both are having a good evening, but only one of them is keeping score in the way the box describes. Quiet gets misread as boredom when it is often just a different purpose.

Before assuming somebody is not enjoying themselves, watch whether they keep turning up. And if you are the competitive one, hold your intensity near the level of the table rather than the level of the game.