Its left arm, that being the one furthest in the foreground. The right arm (ironically to the left) that's mostly just silhouettes is perfectly fine. It's the one that's more in front of the body which feels flat.
It looks a lot better on my tablet than it does on my desktop, but it could still use a little depth, and by that I mean contrast. I did a quick run over the painting with a multiply and an overlay to darken some edges and shadows, while brightening a couple highlights, and tried to separate the arm a bit better. I then parts blurred it a bit, and here's the difference.
Not much, but it helps the image pop and your eyes track toward the focal point of the head better. I may have gone a little overboard with the arm though, since now it has a fair amount of contrast with the background...