Plastering woes

Why does my finishing plaster always crack as it dries? I recently repaired the plasterwork around a fireplace which I have installed. The browning went on fine, then the finishing coat, which I re-wet and polished a few times. All seemed fine, quite a smooth finish for me, then over the next week it cracked and came away from the browning coat. I always try to key the browning and have used pva to give better adhesion. Is it because I am not letting the browning fully dry before finishing?
Thanks, Ian Perthshire
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