Help Needed with FITS Liberator

[Stub Mandrel]

I'm getting really confused by FITS Liberator.

When I first started with it, I found it's mathematical stretches really effective.

Today, it's been putting some really strange gradients and coloured patches on my images. I couldn't see what was causing them so I've stretched my master flat, dark and dark flat and they all look smooth and even, aside from the vignetting and dust bunnies you expect on a master flat.

Stretching an original image in DSS gives me this (note the virtual lack of any signs of nebulosity, even with extreme stretching!):

I now that's pretty horrible, but it clearly has dark, vignetted corners and lots of smudges.

Stretched in FITS Liberator I'm getting this (adjusted to similar size as the other image). As you can see, aside from the stars it appears to (more or less) reversing the greyscale on the background,  so the dark spots are appearing as light patches: