how can we solve our shower problem

If when you use other appliances and if all the waste pipes are connected from these to Shower Waste then your problem is either; 1. Waste Pipe is sloping in wrong direction (restricting flow running away) 2. More likely that you have a blockage building up from where they all meet back to drain. In this case you would be better off re-piping waste and ensuring all clean and appropriate drops are in place for the Waste Pipe. The use of these darin clearers, etc. you can buy will very rarely work and thus you are just throwing your money away down the drain litterally. Bill Pipe Dreams Plumbing Services Bournemouth

You may have a blocked sewer pipe from the house that is backing every thing up this would need to be cleared with proper drain rods You also may need to change some of the pipework it may well be incorrect put the job up on mybuilder.com Kelvin

HI JUST SEE IF THE WASTE PIPE FROM SINK TO SHOWER IS CONNECTED AS ONE.AND RUNS OUT TO GRID. THE SHOWER WASTE MAT BE LOWER THAN SINK WASTE.SO THE WATER FROM SHOWER WILL FILL TRAY BEFORE IT CAN EMPTYSO YOU WILL HAVE A BILLED UP .REGUARDS GARY.

The problem is possibly that the waste pipe flowing from the shower is rising rather than falling, thus causing the water to drain slowly and allowing water from other appliances/sinks to come back through the shower trap. It only has to be rising ever so slighty for this to happen.