I mostly use Airmail (and many other apps) in Full Screen mode, using the App Switcher (cmd-tab) to switch between apps.

When switching back to Airmail, the app does not switch to the Airmail window. I have to switch back by doing cmd-tab immediately followed by shift-cmd-tab to make it work.

(I sometimes have this problem with Calendar too, so might be caused by something else. Suggestions?)