If theyre really stuck you may need to see outside assistance

Being stuck is a really crap feeling. If your teen feels stuck about their school subjects, you can guarantee the motivation to study will be pretty much zilch. You cant expect to teen to slave away over the books if they dont get whats going on.
If theyre finding the subject difficult, its probably a good idea to get them some help.
Depending on the subject and year level, you may be able to help them. If theyre a senior doing calculus, you may have to look at getting a tutor.
If youre not so keen to be forking out money every week for a tutor, you may not have to. One or two lessons may be all that your teen needs to get them on the right track.
One word of warning though. It can sometimes be tricky to differentiate between confusion and laziness.