Shower not draining

I have installed a replacement shower (new shower, tray etc).

The shower tray does n ot always dran properly. Sometimes it empties with a nuclear powered vortex and on other occasions the tray just fills with water.

If I use a drain unblocker (one of those that looks and acts a bit like a bike pump) the shower then starts to emplty.

Any help appreciated.

Some questions to ask you before help can be of any value...

Did you used to have a shower there?

How much of the drain run is new, and how much is old?

What route does the waste pipe take?

What gauge/size/bore is it?

How much of a fall does it have?

What part of the soil/waste system does it connect to?

What are your venting arrangements?

Hi

There was a shower there before - it drained OK
I am using all of the old drain run (it is set in concrete below the floor)
It is 20 years old - plastic pipe 40mm
Don't know about the fall
Not sure what you mean by venting - the shower empties into the pipe like the old one did

When it was first installed it emptied OK and very occasionally still does

Any help most welcome