This can be quite distressing for a presenter and could be down to a couple of things. Obviously, your audience could be bored so you need to check your materials and get someone to give you feedback on your delivery.
In most cases though, it may be that your audience may have not had sufficient breaks or that your presentation may be at the end of the day.
What you can do, if you see people popping off, is to engage them by asking questions. You could also get the audience on their feet and get them doing some exercises. The audience might think it odd, but it will make your session more memorable and more importantly will have reenergised them for your presentation.