must click twice on the dock icon to go to an open app

Switching to an open application that is on another page/desktop space on my iMac, requires two clicks on that app's dock icon. The first click brings the app's name up on the menu bar (gains focus) but does not switch to the app. The second click switches to the application. Before El Capitan it used to go to the application on the first click. It is annoying, any clues as to why this behaviour?