Why can I not invite my allies sometimes?
For example, I had an alliance with the King of Leon. When declaring war on a Muslim duke holding Galicia, I could not call for the king's aid--it's not that he didn't want to join, I didn't get the option to call him at all. He wasn't in any wars at the time, and did not have a truce with the duke in question.
Do you have a liege? You can't call independent rulers to war if you have a liege yourself.