Nozzle is lower in the middle than in corners

[Saltellone]

Hi all,
I just bought the A20M but I'm not able to print correctly. I leveled the 4 corners and they seems to be ok, but when I move to the X center, the nozzle is lower and it touches the plate.
I think it could be that the X rail bending when it's in the middle
Any ideas?

Thank you in advance

IMG_20211223_192334.jpg (193.43 KiB) Viewed 1531 times

IMG_20211223_192240.jpg (159.13 KiB) Viewed 1531 times

[Saltellone]

Solved: it was the X rail that was too loose. Now that it is tight, it works perfectly!