6) Sermons fail when the preacher gets without giving.
In some ways an expository sermon is a dialogue as we invite the crowd to listen intently. Rhetorical questions are often quite helpful. You might say, "Now you're not listening to me now, are you listening?" Ethos is the audience's perception of the preacher. If the flock loves and respects the preacher and if the speaker cares about his flock good things happen. If you show interested in your people and get to know them it will greatly help the impact of your pulpit ministry.
7) Sermons fail when the preacher does not understand the congregation's limited ability to listen.
You may need to start where they are and help them grow and mature. Teach your flock how to listen to expository preaching (note this, this, and this).
8) Sermons fail because we do not know the techniques of legitimate persuasion.
We are not "time share" salesmen but we are ministers of the Master (1 Peter 4:10; 1 Thess. 2:13) . We want people to take God's side on whatever issue or topic it is that we are addressing. Consider the examples of Jesus and the apostles (see Acts 26:28). As long as you do not violate the text you are free to rearrange the structure of the inspired text. An emotionless preacher is doomed to fail. You want to be memorable as preacher.
9) Sermons fail when the preacher does not properly illustrate.
The use of illustrations is an art and can serve numerous purposes (note Jesus' own example in Matthew 5-7). They can illustrate, persuade (the Good Samaritan), illuminate the meaning of the text, recapture the audience's attention, and drive home key truths. Do not just tell a story to tell a story but do use effective illustrations.
10) Sermons fail when we do not speak heart to heart and mind to mind.
You do not want to only say what is in your sermon manuscript. A manuscript often launches the preacher into extemporaneous exhortations, applications, etc. Eye contact is really important when you are calling people to action. "Of all the means of art, preaching is the greatest."