1)
What is the meaning of "Ki Lo Nishma ha'Davar"?
1.
Radak: They did not hear at all the matter that the king spoke to him 1 . Therefore they believed him, and did not investigate more.
2.
Malbim: They misunderstood Yirmeyah's answer, therefore they did not ask what the king said to him. Had they asked, he would not have had an answer for them!
1
Also, they did not hear that the king called Yirmeyah to come to him. Had they heard, they would not accept Yirmeyah's answer!