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I've noticed that my rear derailleur (Ultegra 6700) will not return to where I've set the B-tension adjustment. Pulling it away from the hanger means it will just stick there, or maybe move back a little bit. Comparing to other bikes with Shimano rear derailleurs, mine seems to be odd one out.
It mostly causes annoyance when removing the rear wheel -- I have to push the derailleur back "in" afterwards, instead of relying on it just naturally going back to the correct position.
If I understand you correctly, you don't have a B-Tension issue. B-Tension sets the clearance between the upper derailleur pulley and the chain, and should not be used to add spring return force to a derailleur.
In this case, I believe that the derailleur pivots are in need of cleaning and lubrication, and I would start there.
If that doesn't fix the issue, you can look at replacing the derailleur return spring (#2 in photo)behind the fixing bolt of the derailleur and/or the cage spring (#1 in photo)between the derailleur body and the derailleur cage.
Assuming you have already backed the B-tension screw all the way out, I believe your B-tension spring has simply lost its tension. Mine is the same way. I know that spring is replaceable in some derailleurs, and checking whether it is in this one is on my list of things to do. Check with your local bike shop.
