If I select an instance and release the keys, sometimes the instance doesn't open up.
If an app's icon is shown in the app switcher (invoked by using the keyboard shortcut Cmd + Tab), it indicates that the app is indeed running. However, selecting the icon from the list using the Tab key, and releasing the Cmd key wouldn't always show an app window(s). This could happen in following cases:
All the open app window(s) are minimized: If all the app window(s) are minimized into Dock, using the app switcher to switch to the app won't automatically un-minimize the app window(s). If this is the case, just follow the procedure described below to also un-minimize an app window in addition to switching to it.
After you have selected the desired app from the app switcher using the Tab key, continue holding the Cmd key after you let go of the Tab key, and press and hold the Opt key. Now with the Opt key held, release the Cmd key. If the app has one or more of its windows minimized, this key sequence would un-minimize a window.
No app window is open: It's possible for some apps (based on how it's programmed by its developer) to not have any visible/open app window even if it's running. In such cases, the trick discussed in the first case won't work as there in no app window to un-minimize. You can either click on the app icon in the Dock, or use one of the commands in the app's menu in the Menu bar at the top of the screen to show an app window.
Here's a quick way to quit an app shown in the app switcher. After you have selected the desired app from the app switcher using the Tab key, continue holding the Cmd key after you let go of the Tab key. Now with the Cmd key held, press and release the Q key once to immediately quit the app. The app would also get removed from the app switcher instantly.