Uponor underfloor heating installation

We have a Uponor UFH in 2 zones in the house, up till now I cannot figure out which zone is which.
However recently the boiler has being over working to compensate for the low air pressure in the expansion vessel. I have cooled the boiler down, and didnt have to release the pressure valve to bring the filling loop gauge to zero because the heating system was loosing the pressure. The expansion vessel has being recharged with a foot pump and it is at 0.5 - 1 bar as recomended by Grant Vortex (outdoor pro 26/36) manual.
The stats and controls does not seem to be calling the boiler as often as it should and the temp gauge on the underfloor heating manifold drops to 30 degrees rather than 40 as it should be to keep the floor pleasantly warm. As a result of that the floor gets a bit cold.
The boiler only seems to fire intermittently when called by the UFH room thermostats. The floor does not get as warm as it use to. At one point I thought that we had a leak because the system was loosing pressure, but since I recharged the expnsion vessel and had the system professionally treated for small leaks, the pressure has stayed put. I project managed the whole building project and very certain that the pipes were laid meticulously.
The stats use to stay on until the system is warm enough and cuts out as intended, but this doesn't happen anymore. The stats stay off more than they are on now (orange/red light). The system had worked fine from when it was first installed in 2009.
Can anyone please help? I have called out the heating engineers about 4 times in the last 2 months and simply cannot aford to be calling them out that often. Can you suggest what specific questions or suggestions to make to the heating engineers if I call them out again, or it there something that I can check?
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