I seem to have found a physics bug. I�m working on a Favourite Features entry for revolve-joint, but occasionally the physics breaks. There are several scenarios, but the most repeatable is to spin until the balloons happen to touch, blocking movement. The next spin will result in these balloons not moving, unless they happen to be hit by the third balloon - then they jump to their revolve-joint position and behave as expected.
There are other scenarios where even with no balloon touching any other, they all remain stationary indefinitely.