They will not answer you
Jeremiah's call was not from Israel. Jeremiah's call was not from Judah. Jeremiah's call was not to be popular. Jeremiah's call was not to preach a message that would be accepted and followed. Jeremiah's call was from the LORD, to speak His Word to His people. Here the LORD tells the prophet that his work will not be very "profitable," that is, many people will not obey or answer what he has to say.
"Gee, thanks a lot, God," I can imagine Jeremiah saying. "Thanks for nothing. You call me to this ministry, don't allow me to even marry, put me into situations that humilate me, all this for what purpose? The people that I suffer all this for aren't even going to obey or answer."
But, the redemption of the people was not Jeremiah's job; that's Jesus Christ's job. The redemption of the people is based solely on the gracious work of the LORD Almighty and not the proficiency of Jeremiah's preaching or the answer that he elicits from the people. Just as Jeremiah's call is from the LORD, so also his accountability is to the LORD and the LORD alone will measure his success.
So it is with each of us. "It is impossible to argue someone into the Kingdom of God," is what a college professor once told me. It's not belief or knowledge that saves; even the demons know God and believe in Him. What saves people is faith, faith which the Holy Spirit alone can work. Our job is not to save people; our job is to preach the message given to us by the LORD.