Paint keeps lifting off unintentionally

Sometimes the type of paper you're using can be a factor. It's best to use a thick, 100% cotton hot pressed paper. The more cotton in the paper, the more robust it is and the better it will be at accepting multiple layers. 

Also, when applying another layer, make sure that your first layer is completely dry and that you're not scrubbing too much with your brush as this can lift off previous layers. 

You'll see Anna navigating this issue in some of the tutorials - especially any where she's working in a sketchbook (where the paper isn't quite so absorbent) and the subject is dark. In these situations, Anna may dry the painting (you can use a hairdryer if needed) or use on a very gentle pressure with the paintbrush. Using the right paper in the first place can largely avoid the issue.