Hello Greetings.... perhaps I should say greetings, Greetings
It would seem apparent to me that your crystal skull is willing to work with you if it was manifested something for you once. Perhaps you need to give it some special treatment to encourage it. You might try soaking it in a warm solution of sea salt. That will purify any extraneous energies that might have become attached to it and make it more receptive to your energies.
Let us know if that has any effect for you.
Namaste!
Uriel