When working with natural plasters you need to remember that there are no chemical binders to help the plaster stick to the substrate. In most applications this does not matter; however, when plastering overhead (a ceiling or above a window) this becomes very apparent. In fact, it is not unusual to have more plaster land on your head than sticks to the lid!
Many people talk about simply applying the mud in smaller amounts and using smaller knives to apply it. This may work, but it does not address the real issue. The falling plaster is a sign of a bigger problem. One potential problem is the plaster mix itself. The falling plaster may indicate that there is not enough clay or lime in the mix and the high levels of sand are causing the plaster to fall. Another potential problem may be the mesh or lath that is used in the area of application. Be sure that the lath or mesh has some space behind it so that the plaster can get pushed through and then hang on the metal.
If the metal is tight to the substrate, it will not create the hanging detail that is required for the scratch coat. Once you move to the brown coat, any falling plaster is clearly an indication of a bad mix. Add less sand and create a stickier batch of mud. Be careful not to add too little sand in relation to the clay or lime as then you may end up with a plaster that cracks heavily when it dries. Plastering is an art and mixing the materials is just one detail within that art. Be patient with yourself and learn from your mistakes. You can always add a bit more mud or scrape off what you have already done if it isn’t right. This is, of course, not ideal, but it is an option!