Hi Kimberly:
Good techniques, Kimberly. There are simple, and there are more involved techniques, but all should work to get the attention of students back on track.
Sometimes something simple like telling a story that redirects their focus back to topic is all that is needed. Other times, swithcing to different activities, or using different media, or maybe a lab prcatice (if applicable) may be needed.
Reasons could be the students are tired, bored, unprepared, material is over their head, or they don't feel the material is important. For whatever reasons, students get disengaged when the current activity looses interest… >>>