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 simulate home button double click also works sometimes. I have verified this with a work mate and they have the same issue. I am not sure if this is a bug in the simulator or if there is a setting somewhere on for example the delay between the clicks? The answer from here did not make a difference.
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 <simulator ID> <your app bundle ID>
... or if you have only one simulator running, just type booted:
xcrun simctl terminate booted <your app bundle ID>