cant get windows 10 to switch back to mac os x

I recently installed a full licensed windows 10 home edition through boot camp. I have a MacBook Pro 2105. I cannot get back to Mac OS X. I've tried multiple times restarting/powering off computer and pressing down the option key. It does give me the icon to click to switch to Mac but every time I click on it, it restarts and gives me a symbol with a circle and slash (indicating it can't/ won't perform the function). And the computer still windows mode. I've also tried the icon panel in the bottom right which I can open up the bootcamp control panel. It gives me a choice to restart in Mac or windows. I choose Mac and the same thing happens. When it restarts, I get that circle with a slash and I am still in windows. Help!! Thanks!!