When you are preaching, your listeners are looking for unity (a single focus to your preaching), order (a clarity of structured presentation), and progress (a sense that you are moving forward and getting closer to the end). It is this progress that can be easily lost causing the message to feel like it gets stuck in the mud.
What causes momentum to be lost? Could be one of several things:
Is momentum about content of the message? Yes, it can be. Is one part of the message too dense or extended in terms of explanation? Is there too much repetition that might give the sense that you are losing your way or going round in circles? Content issues can cause a loss of momentum.
Is momentum about structure of the message? Yes, it can be. If you haven’t previewed the structure, or don’t give effective and deliberate transitions, then it can all meld into one and feel dense or still instead of progressing. If you structure your message so that you keep jumping around the text, listeners can lose the sense of progress that comes from a sequential following of the passage. (It can be appropriate to use this approach in a text, but make structure and transitions extra clear.)
Is momentum about delivery of the message? Yes, it can be. If you lose energy, or become monotonous in voice or visual presentation, then momentum can seep away. If you lose your initial enthusiasm (or if your enthusiasm is at a constant high pitch without releasing that tension), then momentum can be lost.
Momentum can be hard to get hold of, but for preaching to engage listeners, we have to consider not only unity and order, but also progress. Don’t take this the wrong way, but they like to know you’re getting closer to being done!