I am not sure if this is supposed to happen, but sometimes when I close an app and try to open it again by double clicking it on the dock, it doesn't open. I have to right click it and choose quit. Then start the app again. For instance, I have Mx player out with a movie playing. I would click the red x circle on the top left of the window. Then when I want to just open mx player by double clicking it on the dock, it doesnt open. If I command+tab, I can see the icon there. Same with Audacity and Logic Pro. I open the app, then click the red circle x to close it. When I double click it on the dock, nothing happens. I have to quit then start it up again. For Itunes and Mac App store, they come up after I click the red x circle, so here it works fine. I am just not sure if there are specific apps that won't open up again unless they are "quit" from first.