New shower not draining properly, maybe design problem?

[RobCardiff]

OK thats one theory gone. The 'flat' bit of pipe- is it actually flat (poor) or is it a bit uphill?

It's not uphill. The pipe goes down at a 45° angle for about a foot then flat for about 4 or 5 feet into the drain. I don't think that's the problem. The trap always has some water in it just below tray level. I don't think that would be there if it was a a problem with the pipe placement.

 

[oldbutnotdead]

It's not uphill. The pipe goes down at a 45° angle for about a foot then flat for about 4 or 5 feet into the drain. I don't think that's the problem. The trap always has some water in it just below tray level. I don't think that would be there if it was a a problem with the pipe placement.

Well here's a thought. The trap will have passed BBA or some Euro equivalent to be on sale. Your flat run of waste pipe would fail building control inspection, as would the 45 degree section. I'm no expert on those traps but they may be sensitive to pipe layout.
You may end up having to bite the bullet and chop a hole in the ceiling below to sort it out thanks to the poor installation.

 

[OvertonPlumber]

The only way to get the traps to perform properly is to run the waste pipe in 50mm from the soilpipe or outside gulley. Normally the last 300mm go down to 40mm. All waste pipe as should fall at 1 in 40. However experience tells us 50mm can be put in with very little gradient.
From a plumbers point of view shower trays are a pain in the ass. They started out at 100mm high then went to 50mm then 40mm high. Latest stuff is 30mm and 27mm.
With these shallow trays you have to accept you will be standing in a puddle when you shower.