Ender 3- Bed level- Centre Sag, help?

[99ganna]

Hello,1st post, and beginner printer, please forgive any naivety. I've printed some stuff that has worked out. Got a new variety+colour of PLA, and can't get past even a bed levelling, test. The corners look ok (not perfect!), but the centre is a mess, and has failed many times. I can't make anything get past the first layer in the centre! Hopefully the image is attached above.Ender 3 V2Heated+Glass BedNozzle temps attempted: 190°, 200°, 210°, 220°Bed: 50°, 60°I don't understand how a glass plate can appear to have a 'sunken centre'. All rectangles have 2 wall skirt that meet, and then create nice rectangles...the centre skirt walls don't meet and then it does what it you see in the photo. Any help appreciated.(Please also let me know of I'm not posting as per forum protocol!)Cheers.