Is there a water trap (dunno if that's the correct term) somewhere on the drain besides the one in the shower? That could create a vacuum like effect, making it drain very slowly. Just fixed a problem like that by removing the shower trap. Our plumber was a bit embarrassed that he hadn't noticed the trap in the basement, when he did the bathroom plumbing.
This is interesting. I think it is the basin (trap) then shower (pretty flush tray) and then bath (trap) so plenty of ways that a vacuum could be formed.
I did think it could be the issue, but I don’t know for sure what’s under the floor.
Is there a water trap (dunno if that's the correct term) somewhere on the drain besides the one in the shower? That could create a vacuum like effect, making it drain very slowly. Just fixed a problem like that by removing the shower trap. Our plumber was a bit embarrassed that he hadn't noticed the trap in the basement, when he did the bathroom plumbing.