Bed leveling on Anycubic Mega

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

sounds like you've pretty much identified the problem.

It could also be a slight dip in the x-axis in the middle - but most ilkely is a non-flat bed

Although if you've got the glass ultra base - it's really unusual to get a glass bed that domes in the middle.

Check the level (with paper) in line with the centre of the bed - but at the front and back of the bed.

If they match up with the centre then it's an x-axis issue (a very slight bend in one of the guide rods could cause this), if they don't and you just have a bump in the centre of the bed - then it's the bed and you should contact anycubic and ask for another bed.
You're not being over fussy. A dead flat bed is pretty crucial and anycubic have a good reputation and should replace it for you.

Thanks. I wrote them and they sent me the link to check flatness and smoothness of all components of the printer. Will need some hours to go through...