In 15.10 everything works fine with the default super+space but when I change it to alt+shift I can change the language from english to the second one but when I press alt+shift again it doesn't work and stays in the second language. and sometimes for setting the alt+shift as a shortcut key it doesn't even recognize these button and recognize them as a different button! and I have to re-open the text entry setting
I just changed Super+Space to Alt+Left Shiftand it works find for changing to the second language and returning to the original one. But as I changed it to Alt+Right Shift, It just worked as you described.
Generally, it is advised to avoid Alt+Shift for this purpose since these keys are being used in some other apps as Alt+Shift+KEY.
I also suggest getting used to Super+Space.