Am I Levelling My Bed Wrong?

Hi all, i'm relatively new to 3D printing and for the most part it's been going well however recently i've noticed something weird going on with my print bed. i've followed several different youtube videos and online posts to level the bed however the problem i've got is that while i can get the four corners to be levelled and print out the filament at the correct thickness, the middle of the print bed seems to brint as if the gap between the nozzle and the bed is too far away.

i've tried nearly everything i can think of and couldn't find anything on the web. I print on Ender 5 with the standard magnetic removable build plate if that has anything to do with it but i'm really not sure :/