Hi Jonners,
I've had the same experience, and it's slightly frustrating not knowing the exact reason for rejection. (It turned out to be a more rigid application of WAB's ruling on the appearance of copyright/name on images, which, in my case, needed to be made slightly smaller).
If you are satisfied that the image complied in terms of filesize, pixel dimensions and correct Latin & Common names, then maybe depth of focus was the sticking point
As I generally concentrate on flower/macro photography myself, I appreciate exactly the problems you will have had with this subject. Short of going down the image stacking route, there's not much you could have done to get round it.
Possibly shooting from slightly further away, and then cropping more would have given a little more DOF, but heavier cropping brings other problems into play. - No easy solution
Regards,
Mike.