bed still not levelled after G29

Hi everyone, I have an issue with my SmartCore (RepRap, Marlin) 3D printer. It has the G29 feature to level the bed according to a probe, but it does not compensate for what it should at the back of the plate.

- I tried to over "de-level" the bed (like 2cm and it still compensates so the G29 command works) but in any case, the back is too close to the head, and the front is fine.
- I replaced every pieces of the X-Y axis yet the issue remains.
- I use aluminium tape under my glass bed for probing.
- It is a Core XY printer (see image attached for more details)

Do you have any idea, even the smallest, to my problem ?
Does an individual offset for each 4 points probed exist in the firmware ?


Thanks a lot guys


pictures of the problem here :
overall printer :


bed from front


bed from above