How many words does your essay have? 800? If so, you might consider increasing it.
Short and concise is good - it saves you time, but I don't think it earns you marks (this isn't english, lol). Also, your essay might not be as "concise" as you think it is. If you're only bringing in 2-3 arguments, then that is not enough. What you want in your essay is content - addressing every point there is. One of my essays had 8 body paragraphs (I think???) - in any case, it is a lot - because I tried to cover every point I could think of (including the random ones that just popped in my head).
That's what eco essays are about. content. content. content. (and that's what markers look for!). I think that's the most common pitfall of most students - only dishing out 3-4 pages is not good enough!
If you don't believe me, check out some of the exemplars from ARC. I opened a random essay:
http://arc.boardofstudies.nsw.edu.au/standards-packs/SP02_15110/ - and it had 15 (written) pages!
edit: perhaps find other exemplars/responses by past students/sample essays from past papers and compare and contrast theirs with yours. That's what I pretty much did to find out what it takes to do well