Bed leveling on Anycubic Mega

[Rwetzel]

Hi;
I just got my Anycubic i3 Mega a couple of days ago, I am a total newbie in 3D printing.
I believe I may have a warped bed or an issue in the x-axis, since when I get all 4 corners leveled (similar resistance to pull the paper from under the nozzle), the paper gets stuck in the center of the table and in most of the (constant X) line at the center. I need then to lower the bed a bit, and I get some resistance in the center but no resistance in the corners.

Attached an image of the calibration squares I printed, showing the center a bit squished (but still ok) and the corners with adhesion issues.

I would like to understand how bad is this issue, meaning: is this really bad or am I being too picky?
Also, what would be the steps to identify the source and solve it (or minimize it).

Thanks!
Ricardo