I just upgraded to Android Lollipop on my Nexus 5 using the OTA update. For some reason, I don't seem to be able to use the multitask button. It works very rarely, but I'm having to revert to pressing Home then opening my apps from there. This never happened on KitKat.
Is this a known issue, and is there anything I can do about it? Restarting the phone doesn't seem to help.
The OS recognizes that I'm pressing the button, since it highlights and vibrates it.
On a related note, I've been having to tap the Home button twice for it to actually go the home screen.