Flooring over 4 inches wide is normally face nailed. If it was only blind nailed leaving the windows open in typical summer weather is enough to explain the cupping. Adhesive may've worked if it was glued directly to the subfloor or to at least a 3/8 underlayment. Going back and face nailing the floor should flatten it out. The thinness of the floor should make it easier to flatten but it will still be a lot of nail holes to fill. Adding another coat of finish may slow the seasonal movement in the future.