Your advice please. I added a comment to the pastering forum about my doorway. it has "popped" on the last coat of render. it has got a hollw sound to it when it is tapped with a metal object. a close inspection reveals an are...

I am a newbie to this site and plastering in general and I recently did a complete hallway/stairwell, only ever having done patch repairs before this. I boxed in the stairwell drop and even boxed in some pipes and electrical conduit. No-on...