When an object is attached to an avatar llTargetOmega does not seem to update.
Now, it works if the object is recompiled while attached, but without recompiling the object keeps it's Omega (at least from the client view) and ignores other llTargetOmega calls.
You can force the object to spin correctly by right clicking and then deselecting the attached object. However, that's not really that great a solution.