I got a Motorola Google Nexus 6 phone which takes a nano SIM card. I cut down my existing SIM as far as I could and it worked in the Nexus but the SIM tray wouldn't shut properly so I ordered a brand new nano SIM.
When the nano SIM came I could not get it to work at all. The phone would not detect the presence of the SIM card. I tried wedging paper under it, cleaning contacts, factory reset, software downgrade and nothing would work.
This same nano SIM worked perfectly in another handset, I tried another working nano SIM in the Nexus and that wouldn't work either.
I then tried pushing the SIM tray as hard as I could and surprisingly the SIM was registered by the handset and it indicated full signal.
Push the SIM tray in as hard as you can!!!
This appears to be some kind of mechanical defect with the Nexus 6 as recorded in many threads.
And Here: https://productforums.google.com/forum/#!topic/nexus/-vSXPn2obNk