This seems to be a complicated problem so I am going to do my best to explain the issue, and how I got here.
When I open the Android Device Manager on a web browser using my main Google account, ADM says that I have "no active devices", however I have both Nexus 6P and a Nexus 7 (2013) both running the latest Marshmallow March security update.
When I open Android Device Manager on either device, the panel that lets your select your device never finishes loading and will constantly show a loading circle.
I also have my work email (a Gmail account) on my phone, and if I sign in to Android Device Manager on my phone or the web I can locate my phone just fine, so it clearly is an issue with my Google account.
What I have tried:
Ensure location is enabled
Ensure all device administrator and Android Device Manager settings are enabled
Device is not set to "hidden" in my Google Play settings
Clear data on Google Play Services
Remove work email from phone
Upgrade to Android N preview
Downgrade back down to latest Marshmallow release
Another odd thing I noticed is that the new Android Beta page for enrolling in the Android N preview does not show my Nexus 6P while using my main email, however my work email does show it. I even called Google support yesterday and troubleshooted for an hour and by then end they said that they were resetting something to do with my location which should take 15-30 minutes, and they would call me back. I never got a call back, so I am avoiding calling them until I seek more help.