hhmmmm...After digging around a bit, I have an idea that it may be my biceps (long head).

Does anyone know good stretches for that?

If it's the left arm down that is bothering you, i'd say more often than not, it's actually b/c of L shoulder tightness that is inhibiting the rotation backwards.
My Question for you:
where do you feel the pain, and what kind of pain is it? (sharp and pointed? achey? does it feel just tight and inflexible? Or do you feel pain in your joint?)

Obviously it's challanging to "diagnose" over the computer, but i'd love to try to help you if i can.
I'm a yoga therapist, massage therapist and health counselor, as well as a bellydancer. And I'm kind of a geek about injury stuff like this, especially b/c last year i separated my shoulder so i'm keenly aware of how it is affected in bellydance and yoga!

My guess would be that something around your shoulder girdle is really tight. Could be your pecs, your traps, yes your biceps or your traps (in your arms) but my guess is that more often than not it's tightness in the shoulder and surrounding muscles that are limiting the range of motion or mobility in the shoulder flexibility that would inhibit you being able to bend your arm fully.

Suggestions:
1. Do the other arm first, so you get a nice stretch in that Left lats, triceps, etc.---the whole side and armpit area.
2. Do shoulder circles backwards to loosen up your shoulders before doing the pose.
3. Clasp your hands behind back, on the inhale lift your chest and stretch your knuckles up the back wall to open up the front of shoulder and the pecs--on the exhale, bend forward at the waist, taking your arms up over head behind you.

THen try the pose again.

There is so many therapeutic things I could show you to do for it, but have to be in person (obviously.)
Try those and let me know if they help at all!

Also try this:
opening up your neck/pecs/shoulders by:
placing your L arm behind your back as you would for the pose.
then doing gentle L shoulder rotations backwards. Small. to open up the shoulder.
Then, keeping your hand behind your back, take your R hand to your L shoulder--anchor a point where it feels tight with your R fingers. Then turn your neck away to the R, back to center. To the Right, back to center, etc. It's a combo of self-massage and yoga stretching to really open up tightness in your neck and shoulder.

does that make sense?
And get a really great bodywork session. :)
good luck!!!

Also: i just saw this too: www.yogajournal.com/poses/689. cow face pose.

It may be as simple as that sometimes, one side is stronger than the other. Sorry to sound trite, but that's how bodies work,....sometimes.

i have the same problem on the other side...i can reach my left with my right when my left is on the bottom but the reverse has me barely able to rotate my right shoulder enough to get it near the middle of my back in cow face pose. for me, my right shoulder just does not rotate enough yet to reach.