Hi Chris:
Students lose interest and need some reengaging for several reasons: tired, bored, not interested, lost their attention, can see no benefit to them (WIIFM, if you know that that is; "what's in it for me?").
If we can identify what the primary cause might be, then we have a clue about how to deal with it. One thing I think works effectively most of the time is sharing a personal experience or telling a story. People like stories, maybe because they can relate to the situation or characters , or I don't really know, but stories always work. And you don't need to take a break, necessaarily. You just merge the story back into the lesson at hand.
You can be more agressive too, by either changing the method of delivery, or changing activities altogether. Some days, the more agressive is absolutely needed; end of the week, following tests, where cramming has precede the testing), Mondays, etc. So just need to be a little creative, try to see things from the point of view of the students, and the'll get back on track.
Regards, Barry