Your 'best' is a good example of background that help show your model it has vignette , soft gradient , something to break the flat color ,ground shadowOn the other hand 2 new pieces have not particular appalling color [think they call this muddy colors then you mix complimentary colors they became mud if you have to keep…
I really do think that you should approach artists at the studios you want to apply to for feedback. Besides having clear demarcation between models (game res models and pure sculpts), I don't see anything critically wrong with you work that would affect a job application. The way the market is now, I wouldn't be surprised…