All the prophets do not prophesy whenever they wish, but rather focus their minds, seclude themselves, and seek prophecy... and even though they focus their minds, it is possible that the Divine Presence will rest upon them and possible that it will not.

196. The Sephardic and Ashkenazic Dispute on the Blessing for Tefillin Shel Rosh מחלוקת הספרדים והאשכנזים על הברכה לתפילין של ראש