I am trying to edit a multicam clip but I'm not having any luck. I've created a multiclip and when I move it into the sequence it is unrendered. All the other clips in that sequence are the same codec (xdcam ex 1080p25) and are playing fine. I created a new sequence and set the codec to xdcam ex before dropping the clip into it and the clip still shows as unrendered. I have no idea what's going wrong, can someone please help?

Is your RT settings on Unlimited RT?

It is now... Crisis averted! Thanks Mathieu, much appreciated. I still don't understand why this particular sequence needs rendering while others don't. Is it because I'm using 9 angles for a change instead of 4 or are there other factors that effect the playback performance of the video?

That's exactly why. Your computer is having a hard time decoding so many XDCam files for the multi clip function. As long as you are able to do your edit in UnlimitedRT, all is well. Otherwise you would have been forced to transcode to ProRes.