With the iOS simulator version 10, double clicking the home button to bring up multitasking (running apps) sometimes does not respond. Even hitting (cmd + shift + h)x2 to simula
I have this problem too when I want to close my app. So, I use command line instead:
xcrun simctl list | grep Booted
then copy simulator ID, and run this command to close app
xcrun simctl terminate
... or if you have only one simulator running, just type booted:
booted
xcrun simctl terminate booted