Ender 3 build surface is sticking up in the middle I have had my Ender 3 for a few months now and have loved it! I recently noticed that the build plate, the black removable part of the print bed, is sticking up in the middle. If I poke it, it goes back into place, but will not stay there. It shows especially when I print my leveling squares and find the the corners are near-perfect, but the center is crazy thin.

I am using a stock Ender 3 with Cura 4.5.0.

For those who may find this to be helpful, I am looking to find a good replacement build plate. In the meantime, I managed to get mine (without the sticky back) to lay flat by increasing the temperature of the bed from 50 Celsius to 60.
So your (clone) BuildTak bed surface is not sticking to the bed anymore, time to remove the surface and clean the heatbed and apply a new one. These build surfaces (usually) have a sticky 3M sticker to stick it to the bed (or the surface bottom is covered with a sticky surface). When this sticking layer fails of subsequent heating cycles you need to replace the build surface. The higher the temperature the more the build surface expands the higher the surface gets when it doesn't stick in certain places. 

Note that the build surface is a consumable, it needs to be replaced once in a while.