I have the Creality CR-10S. I replaced the glass it originally came with as it was warped and I wasn't getting any material out on 2 of the sides.
I switched to a PEI sheet and got the same results after only a few prints.
I then switched to a mirror I had cut previously, and it worked for maybe 2 days. I scrapped a print off that warped, and then the leveling was off on that. My gantry is level, my beds aren't warped, I'm leveling it to allow a standard piece of paper to have friction by the head. It's either to close and no filament is extruded or it doesn't stick at all and drags filament everywhere. If I can get good extrusion on one side, it's horrible off on the other or all corners.
Note that I have checked my gantry and it is straight as well and my the bed. Not sure what issues I have left.