The lectionaries used in Lutheran churches don’t cover the whole Bible, not even the New Testament. Doesn't this contradict the instruction to preach God’s whole counsel if so many Bible passages are never preached on?
If Lutheran Churches follow the ILCW three-year lectionary series, they will offer readings from 57 of the Bible’s 66 books. That is a good amount of variety. Of course, when pastors preach on one of the readings, they will very likely refer to or quote other sections of Scripture. That means that worshipers will be on the receiving end of even more portions of Scripture than the readings themselves.
I am not aware of any lectionaries that cover the whole Bible (all the chapters). And that is really not the point of the Scripture passage to which you allude. In Acts 20:27 the apostle Paul told the Ephesian elders, “For I have not hesitated to proclaim to you the whole will of God.” The King James Version had translated that verse as: “For I have not shunned to declare unto you all the counsel of God.”
The point the apostle Paul was making was that he had taught God’s revealed will, his will as revealed in Scripture, to those Christians. He did not cover up or hide any truths of Scripture. That does not mean he read the entire Old Testament to them.
So, using a lectionary does not in any way contradict what we find in Acts 20:27. There would be something wrong if our pastors were holding back from teaching what God’s word says. That is not the case. They are proclaiming the whole will of God.