My deck looks like this after it rains.
One board and parts of others are damp for a long time after everything else has dried. What could cause this and is it hard to fix?
My only guess is that the board is upside down relative to the others.
It looks like that board may be right under the edge of your roof. Do you have gutters installed? It could be that whenever it rains that board is constantly saturated by roof runoff.