My clips sometimes export dark

Out of nowhere, I have been getting clips exporting, both .mov and MP4 (both H264), quite a lot darker than they are in resolve - sometimes very much and unacceptably darker.
I have not colour corrected them, it just seems to do it with some projects but not others.
What could this be?

thanks

How one views the image matters. Computer monitors and software players aren't the most reliable.

So as a simple test, look at the scopes of the original image, then bring the exported back in and compare those scopes.

Report back.