Radon Levels Remain Elevated
A gas or oil burner in the basement could produce a small vacuum if that burner takes the combustion air out of the room where it is installed.
If the basement windows and doors are closed, the radon could be sucked into the basement via cracks and openings in the walls/ground slabs.
Newer burners do get the combustion air through the exhaust pipe (counterflow in a ring).
Also other equipment could provide a tiny vacuum in the basement, f.e. fans blowing outward.
The layer(s) just below the first solid building layer and next to the walls under ground should always be made of material to break the capillary effect, f.e. small pebbles. This will avoid water getting sucked into the basement.
The radon can free flow in these pebble layers, and one way to reduce the radon level is to suck the air from this layer and blow it to the Lee side or over the roof of the building.