: The only cause for this as far as I can see is wood shrinkage. We
: have had an exceptionally dry start to the year,
: with no measurable rain for the past two months, so the atmosphere
: has been quite dry as well.
: All of the wood was dry when I was building but I recognize that
: there is always some moisture present.

How about running a humidifier in the shop for a few days, see if that helps the wood come back up? Easy way to do this would be to soak a few towels, and hang them to dry in the space, refreshing with wet towels as needed.

StephenHJ