I think Dave (Brundlefly) may be on to something since the audio metronome is a plugin of sorts... 
 

I have the metronome issue too (since installing 2016.03). Haven't had a chance to test the workaround yet

Finally got around to checking last night and mine works fine.  2016.03 version, but if there is a .71B (which I saw mentioned in some thread) and it represents an update from the Early Release .71, then I don't have "B" installed" if that makes a difference.

I ran into this problem on Platinum 2013.03 this evening.  The metronome stopped working during an active session.  As suggested in posts above, toggling the global FX button fixed the problem.
 
Not sure of the sequence when the metronome stopped working.  I had disabled all FX, and changed the metronome sound under Preferences, and did some playback with the metronome enabled (metronome on for playback and record).  I likely saved the project during the process.