Hello everyone

a few months ago i bought a BLTouch sensor ,installed it on my Prusa i3 (geeetech pro b) and flashed the new firmware to enable it (Marlin 1.1.9) without having big problems.
From firmware I enabled the auto bed leveling function in linear mode specifying that 9 points of the plan must be tested; I also set the various x y and z offsets between the nozzle and the probe.
Made a few small prints (about 5 centimeters wide) adding G28 and G29 to the start G-code and had no issues, the z axis was moving while printing to compensate for the not perfectly level surface

Today I tried to print an object with a much larger surface and the problems started here:
during the printing of the first layer, it was clearly visible that the thickness was different between the various areas of the printing plate,
in some areas the nozzle was too close to the glass, preventing the right amount of filament from coming out while in other areas the quantity of filament was perfect.
To make sure it wasn't a random problem, I re-launched the same print 3 times, always getting the same result, so I decided to change the firmware settings by increasing the calibration points from 9 to 16, carrying out 3 tests for each point of calibration and calculating the average value, also I also added a small delay at the beginning of the test at each point as recommended on some forums.
I then launched the print again and got the same result

What am i doing wrong?
Any help?

Regards

Alessio