No, I don't agree. There will always be some conflicts in style and some people might not appreciate the way I teach and therefore drop out. But if it is a few people who drop away, here and there, and not a mass migration at once, I have my doubts with the idea that the root of the problem lies with me.
Adult students have to be satisfied with what they believe they're getting from class. After all, they likely are sacrificing something else in order to attend. When the sacrifice seems to no longer be worth it, they make another conscious choice.
I agree with this, but I don't think the blame always lies with the teacher. Knowing my own reasons for dropping a course, they normally had little to do with the teacher. Reason 1: The course demanded more time from me than I had originally thought and I wasn't prepared to give more time. 2. The curriculum was set (not by the trainer but by the institution) and I didn't like it. 3. The levels in the class were too varied for me to feel I was making progress. 4. I had missed a few classes due to other commitments and didn't feel I could catch up. (Please don't get the idea I drop out of everything! This is over 20 years that I'm taking this exerperience.)
It may not be worth the sacrifice anymore for the students, but the reasons can be varied.
As for encouraging attendance, well, to be honest, I would be happy if a class of 21 becomes a class of 12 because I feel it's more effective, but I see your point. I would give interesting homework assignments, meaning not a page of grammar exercises to do, but things that intrigue them. For example, (trying to tie this in to a Business English theme) after teaching a class of electrical engineers the passive (the tea is poured) I would set for homework that they have to explain next class a process at work or a how a machine they use at work functions. For them, that's an actual, practical use of what I taught them and they might be interested in what the other participants will say. For a class on comparisons, I might make them reasearch if their city has any partner cities somewhere in the world and then next class compare the differences between the two. Another thing is to let them know what you are doing next week, so they have something to look forward to. Obviously, phrasing here is important, as saying "We will look at the past simple vs. present perfect" isn't going to stir them as much as "We are going to talk about the past next week, so everyone should bring a picture of themselves from when they were young to discuss." If you say the first, then I do agree with Larry, that perhaps you should re-evaluate what you do in the classroom. Otherwise, I think it's up to the participants to take responsibility themselves for their attendance.