If you've had them skiing or snowboarding for several runs, they may have become Fatigued, so check their moodlets. They'll refuse to keep going until that moodlet wears off. Also, if you've had them going continuously, and they stop, but they WILL go again if you tell them- they probably have an injury. Being injured already makes them likely to become injured again, and those injuries stack and can be fatal, so be careful!