One of the thoughts and temptations that will cross a pastor's mind is the doubt, the fear, the concern that simply preaching the Gospel will actually "do" anything. When it comes to sermon prep and review, seeing the Gospel, sometimes the thought crop in, "I'm bring this up again. . . won't it seem tired and old? Really? Resurrection again - aren't you belaboring the point. It won't do anything for them, they already know it."
This is the root of all forms of legalism that have arisen in the church. Pietism - they know the Gospel, so hit them with the Law to make them better. Social "Gospel" - they know about Jesus, but brow beat them so that they will make their communities better. Even old fashioned monasticism - to hear and live your normal life isn't enough, you must join this place to make yourself better.
What these ultimately attack are the efficacy of the Gospel - they attack the idea that the Gospel does things, that it does what it says -- and in particular, it attacks that the Gospel which proclaims "You have life in Christ" actually gives that life.
And why would Satan attack this so? Because if we think there is no life in the Gospel, we will turn to try to find life the in the misleading dream of the Law. And the Gospel then will no longer be our focus - and those people who knew it won't hear it anymore, and they will be focused more and more and more upon the Law.
Of course, the bigger danger for pastors is not that you are actually worried that your people are bored with the Gospel - it's that you yourself become bored - you yourself think, "Okay, we ought to get this by now" -- forgetting that preaching is not simply the giving of information, but proclamation. Yes, we know - but we need to hear. Life in the fallen world has happened - people need the Life of Christ for them proclaimed - your pep talks are worthless, for it is the Gospel of Christ that is efficacious.
Tire not of preaching the Gospel! It does what it says.