I think if I saw someone starting to fall behind I would first take a look at the situation. I would look at the person and figure out if they normally have focusing troubles or is it out of the ordinary for them? If it was weird for them then maybe something is wrong and I would try and talk to them on a personal level first. However if its common and it happens on a normal basis then I would figure out a way to motivate them and get them back on task. Considering every person is different this approach has to be done in different ways depending on the person's personality which you can take the time to figure out. I would offer any assistance I can and just guide them back on track.