Unrendered footage won't display.

Hi,

I'm editing 1080 30p ProRes files in a 1080 sequence. In the past, I have been able to then create a 720 sequence, and work with the 1080 files in order to crop the shots a little.

Just recently though, any of the footage I try to put into a 720 sequence won't display in the canvas or viewer until I render it: the canvas is just a black screen. This is true for both scrubbing and playing, the screen remains black, I can't even see a still from the footage I'm working with.

Usually, the footage will play fine unrendered, just at a lower quality.

After rendering, the footage displays, but this makes it very hard to work with the footage.

Any thoughts? I've tried trashing the preferences and restarting, didn't help.