I had a new bathroom fitted last year into the old box room, and everything has been working fine up until a few weeks ago. The shower is not draining away, I have thought it might have been a blockage so have done everything in my power to clear it, but nothing seems to work except the good old plunger. When it's plunged nothing comes up and you don't have to try hard, it just seems that it needs to get a vacuum under the water to get it to clear, and once it starts to drain there's no further problems with it until the next time it's used and then it's the same problem again.
The way the plumbing is set up is that the drain from the shower goes under the floor to where it joins up with the sink and then it feeds outside to the pipe which then just goes off into the sewer, the connection to the pipe outside is where it's on open ended pipe that feeds into almost like a funnel.
The sink is draining absolutely fine along with all the other plumbing in the house.
I'm thinking it has to have something to do with air, but due to how the pipe is outside, there is no reason that I can think of that it could be causing it.
Any advice would be great, thanks.

Was the shower drain properly vented ? Is the sink and toilet draining OK ?

First I would call the plumber who install the shower plumbing. He will know the best what is wrong with his installation. If he is not available, I would call a professional to inspect the drain with sewer camera to make sure there is no obstruction, sharp turns, bad transition connection from shower to existing house plumbing and/or wrong pipe size used along the drain run.

Back to you / Milo

Hi Jenic

You said, "I had a new bathroom fitted last year into the old box room, and everything has been working fine up until a few weeks ago"...

The first thing I would do is remove the shower strainer and go fishing with a metal coat hanger... could just be soap scum/hair built up in the drain (especially if you have a custom tiled shower as cement can get into the drain during installation).

Try that and report back, OK?

Mark