Just like a PC I do a factory reset of the device, re-pair it and test as-is without any added apps. Then if it fails then the real trouble. Who is on the hook to fix it?
I have a Nexus 6P phone. It has been working fine bluetooth-wise with my car SYNC system until just recently. Now, after my phone screen times out, I can no longer activate FF or RF during song playback using Music Player or Amazon music. Turning my screen on allows commands to the phone, but turning it off disables them. I think this started after I received an update from Google for my Marshmallow OS a week or so ago. I also installed an app called greenify to help save battery, but disabling that app doesn't change anything. Greenify was installed before the OS update so I don't think that's the problem.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic