No video playback but frames display fine

[JasonSoundDesigner]

Hey all,

My video player has decided to stop playing back.

I can import video fine, thumbnails populate as normal, frame rate matches project, DNxHD codec and I can even click and jump about in the timeline and the video window will display the frame at which I've clicked with only the expected lag of a second or so.

But when I hit play the video window stays on the first frame and doesn't move!

No spikes in CPU, no severe lag in audio, no smoke coming out of my mac. It just doesn't play back.

I've deleted my pro tools prefs.
I've tried playing different videos with different codecs.
I've tried playing video from my HD instead of my external media drive.

Cant work it out. Any ideas??

Thanks in advance

iMac 2015
3.2GHz i5
24GB RAM
OS X 10.11
ProTools 2018.7