Divrei Chaim asks this question:
… The gemara (Meg 14b) asks how Chuldah could prophesize while Yirmiyahu, who was a greater navi, was around. (I’m not 100% sure I understand the gemara’s question – if G-d starts speaking to you, how can you not prophesize, regardless of how much greater others may be? I assume what the gemara means is that receiving nevuah requires some kind of prior preparation. If there is someone greater already acting as a navi, that bars others from making those necessary preparations to receive nevuah themselves.) The gemara answers that since she was a relative of Yirmiyahu’s, he was mochel. It sounds like there is a din similar to that of moreh halacha b’makom rabo with respect to nevuah. Maybe this is what was bothering Yirmiyahu – as a talmid, or apprentice navi, he was in a similar position as Chulda and al pi din had to defer to the greater navi.
Answer: I think it’s not about getting the prophecy but about giving it over to others, just as I may “prepare” to decide Halacha, but not actually do so in my Rabbi’s presence without permission.