This can have several causes. The most common is lack of adhesion. This can be caused by using glue that is not mixed well, by air bubbles trapped under the mask during application, or even by contaminants on the substrate. A mask that is properly adhered to the glass will reduce the incidence of blow-offs. Another cause is that the air mixture in the sandblaster may be too lean. This increases the power of the air coming out of the nozzle, which can literally blow the mask off the glass. Increasing the flow of abrasive media slightly may reduce mask blow-offs.