Plaster is picky with how it is applied and dries. Cracking can be caused by coats that are too thick or thin, moister loss, and uneven drying. Minor cracks can be fixed with a gap filler and some paint, but sometimes cracks can be too large to patch.
When plaster shows evidence of cracking and flaking off, it is usually due to moisture loss after plaster has finished hardening and drying. If plaster has been put on too thickly, this can also cause cracks and gaps to form. However, these types of cracks can often be filled with a small gap-filler and then painted over.