4.25" wide 3/4" solid ash flooring installed in June. Adhered to all installation standards - felt underlayment, acclimated for 2 weeks, 1/4-1/2" expansion gap, used 2" cleats with bostich nailer and 16ga nails for face nailing. Humidity since install has been 30-40% (40% in summer, 30% currently in Winter). House is 20 years old with 3/4" plywood subfloors. No moisture problems at all (wonderfully constructed house).

On a few runs up against interior walls, I have some slight buckling; these boards - the last two rows - were face nailed as per standard. The buckling isn't particularly visible - but if you step on the boards, there is about 1/4" of play up and down.

This didn't exist in the summer, and I'm scratching my head since the humidity is only 30% currently. Any ideas on why this is happening, and how it can be corrected before it gets any worse?

15 ga 2" nails. mark and nail on the joists.

Dew point being reached around exterior perimeter walls?

runs up against interior walls, I have some slight buckling;