The tip can be too loose for the Crayon to work even though it is still attached and looks the change. So if your Crayon suddenly stops working, but it shows power when you tap the button, try twisting it counterclockwise to tighten. It may just start working again. More troubleshooting tips here.
The fantastic Matthew Pullen provides a terrific tutorial on iMovie for iPad.
Stop motion is actually a really easy thing that everyone could integrate into their videos. Here, I start out with a broad overview of where you could start and then where you could end up. I also provide a really easy way that everyone can create great stop motion videos for cheap!
The Apple YouTube Channel is super - here is a quick video on how to use the Files app on your iPad. There is also a great hardware/software support site.
https://www.youtube.com/channel/UCYFQ33UIPERYx8-ZHucZbDA