Why can't I pin a screen?

[JillB]

Hi. I'm using a Nexus 4 on 5.0.1, not rooted for the moment but the bootloader is unlocked. I've just tried to pin a screen, but the pin icon doesn't appear in the overview menu after I open the app I want to pin. I tried disabling the functionality, rebooting, then re-enabling it, but that icon just doesn't appear. Can someone tell me what I'm doing wrong? Thanks for any help you can give me!