From what I understand, Melkor aka Morgoth lost a lot of his power as it was dispersed into his creations. This is from Morgoth's Ring:
So Aulë created the dwarves. Did he have to disperse some of his power in order to create them too? I know that Eru was aware of their creation and adopted them as his own but Aulë did the initial 'creating.'The war against Utumno was only undertaken by the Valar with reluctance, and without hope of real victory, but rather as a covering action or diversion, to enable them to get the Quendi out of his sphere of influence. But Melkor had already progressed some way towards becoming ‘the Morgoth, a tyrant (or central tyranny and will) + his agents’. Only the total contained the old power of the complete Melkor; so that if ‘the Morgoth’ could be reached or temporarily separated from his agents he was much more nearly controllable and on a power-level with the Valar. The Valar find that they can deal with his agents (sc. armies, Balrogs, etc.) piecemeal. So that they came at last to Utumno itself and find ‘the Morgoth’ has no longer for the moment sufficient ‘force’ (in any sense) to shield himself from direct personal contact. Manwë at last faces Melkor again, as he has not done since he entered Arda. Both are amazed: Manwë to perceive the decrease in Melkor as a person; Melkor to perceive this also from his own point of view: he now has less personal force than Manwë, and can no longer daunt this gaze.
Either Manwë must tell him so or he must himself suddenly realize (or both) that this has happened: he is ‘dispersed’. But the lust to have creatures under him, dominated, has become habitual and necessary to Melkor, so that even if the process was reversible (possibly was by absolute and unfeigned self-abasement and repentance only) he cannot bring himself to do it.