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Bought the house two years ago. We have the same cleaner as the old owners and she said it's always been bad. I've put every manner of un-blockers, bicarb of soda, coil from inside and it improves a bit but never clearly getting rid of a specific blockage that 'fixes it'.
Every month or so I will have to plunge the shower as it's not draining as fast as it is filling up.
When it was really bad I got Homeserve out who used a coil from outside, plunger inside and a wet vac inside and out. Vac did the most, but again it wasn't a eureka moment.It's a fully slate tiled floor, kitchen extension below, so I'm trying to ascertain if there is a specific cause that could be rectified by putting holes in the kitchen ceiling.
cjr-
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.
JesperXT
Have they always drained slowly? From new, do you know?
Forgive me if I’m missing something, but the answer is probably that there’s a blockage. Rodding the drainage run will tell you the same as an endoscope, and a rod will be more available and cheaper than an endoscope.
https://www.diy.com/departments/drain-coil-with-worm-screw-l-5000mm/5059340003061_BQ.prd