I don't think there is enough contrast between the background and Sydney. All I really see at a glance is tits, face, and 2/3rd's of the title, split in the middle. Everything else is a schmear of black and dark.
Your other two covers were better technically. This one makes my basic commercial art training cry.
I expect the only sales this cover is going to garner would be from the fact it is a Danger Girl book, which means people loyal to the series.
The only way I can see to improve this cover without redoing the whole shabang is to highlight Sydney's outline somehow to bring her up out of the murk that is the background.
I'm not here to bag on your work Boss, the final composition just really got commercial artist training in me into a fervor. And I felt it better at the time to give what in my opinion is an honest critique instead of just passing the piece up entirely.
But hey, your the professional artist and I'm still a hobbyist. In the end if the customer is happy, that's all that really matters.