Back in Autumn 2018 we had our garden " done". We employed a local landscaper to get rid of all concrete, ( the garden was formerly a car park !) to lay sandstone paths and lawn. He also suggested and installed sleepers in front of the sitting room window ( sides and front only ) as a flower bed.
Since then , our grass has been reseeded several times - by us- as it is a soggy mess, many of the sandstone flags have cracked and can be felt moving when walked on and the "trough" at the front window has caused damp to the wall.
We paid substantially for this work. Is it unreasonable to be disappointed in the work carried out?
It would appear that lack of drainage is the main problem.
I think you are probably right about the drainage, but I doubt whether the landscaper would want to know now as it's getting on for 5 years ago since they were involved. They may not even still be in business.
This was my job of 25 years + until a few years back.
You said the garden was a 'carpark'. Any decent carpark will have a had a very deep, solid base - maybe 18 inches of hardcore if it was commercial. If he has just taken taken the 'top' off, and added maybe 6 inches of soil, then turfed it. You are going to get what you have now.
If you can get hold a long builders bar, or maybe 2 ft drill bit/chisel and bang it through the soft stuff, and see how far it goes before hitting something solid. Do this every couple of yards all over the garden, and this will tell you whats been done as regards 'prep' for the topsoil.
If there is a good amount of 'soil' under the grass, it then depends on how its draining/type of soil. This is a different issue. I would check the soil cover first.
The paving stones themselves rarely crack under foot traffic - especially in a domestic setting. So something is wrong there. (very cheaply bought?). Actually moving means they are not fixed on any kind of firm bed. Which is just poor installation.
If the sleeper bed is above your DPC, damp is pretty much guaranteed. This is not 'garden' rocket science. It just shouldnt be there, and wants removing.