In my years of training men to preach and teach publicly, I have come to the conclusion that sermons tend to fall flat for two main reasons:
(1) They don't guide the congregation into the depths of the biblical text, but instead "hover above the text" (in John Piper's words).
(2) They don't directly address the hearts of those who are listening, but instead hover above the congregation.
Some sermons may fall flat on both counts, but failing in one or the other will hinder your ability to minister the Word of God with effectiveness. If you preach or teach the Bible in public settings, your goal must be to avoid "hovering." Make sure you land on the biblical text while also landing on the hearts of your people with its power. How can you accomplish this?
Landing on the text is the result of diligent study, probing the details of each verse, sentence, clause, and phrase, and exploring their interconnections, context, and wider place in the overall teaching of Scripture. Far too many sermons make surface-level observations about a biblical text that any person could have gathered with ease simply by reading it. If you are not telling your congregation more than they can gather by simply reading the passage, you are not preaching. John Piper's "Look at the Book" series is a model, not for sermons, but for giving close attention to the biblical text as you read and study it. Watching that series regularly will help train you in becoming a better reader of the Bible, and thus a better preacher. Aspire not to hover above the text, but to land on it with powerful insights that your congregation likely won't get on their own simply from reading the text.
Landing on the hearts of your people likewise requires the diligent effort of framing your sermon and applying your main points to actual concerns in your people's hearts and lives. Many preachers tend to fall into the well-worn tracks of familiar phrases that have lost meaning and power to most Christians who have heard them for decades. Sermons packed with these phrases tend to fall flat, but sermons carefully aimed at actual sins, actual patterns of thought, actual temptations, actual struggles that our people experience are sure to land with greater power. One of the most helpful concepts in sermon preparation that I have found is what Bryan Chapell calls the "Fallen Condition Focus," or FCF. The FCF is something about our condition as fallen human beings that the particular biblical text we are preaching addresses in a redemptive way. It may be a sin, a weakness, a struggle, or some kind of affliction that is the result of living in a fallen world. Whatever it may be, Scripture points us to God's redeeming grace in Christ to address it. Using the concept of the FCF (of which there may be many, and they may vary from setting to setting), I craft sermons that are framed in such a way that address a particular need in the hearts of my hearers by drawing attention to their need before expounding the text.
Here's an example: A sermon on Romans 3:21-26 may go in any number of directions, but one FCF that would be appropriate for most settings (maybe all) would be our tendency to rely on our own efforts to justify ourselves. A skilled preacher would lay out that FCF in a vivid way: you see this tendency in virtue signaling on social media, in believers who drift away from God because every encounter with him is shrouded in a vague cloud of guilt, in young people who are driven entirely by concerns to fulfill their personal dreams as a way of justifying their existence, etc. The biblical text speaks powerfully to this natural impulse and addresses it with gospel truth about the righteousness of God being given as a gift to those who believe, freeing us from the slavery of never-ending attempts at self-justification. A powerful sermon will address this powerful truth to the point at which people struggle with it rather than with phrases and terms that have lost their meaning and now seem, for most people, distant from reality.
Don't preach sermons that hover in either direction. Put in the work to make sure they land on the text and on the heart. Look at examples from Puritan sermons (the most famous of all is probably "Sinners in the Hands of an Angry God" by Jonathan Edwards). The Puritans were masters at this craft, and we have much to learn from them.