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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.