Why is the center of my glass print bed lower than the corners?
This seems to be a common problem with ender-3 and cr-10 printers from Creality. Mine is the same way but not enough to keep prints from adhering. 

Typically the aluminum bed is not perfectly flat. If it’s not the glass may be able to flex enough that it can make a difference. There are a few ways to try to fix it. 


Shim the low spots in the aluminum bed with aluminum foil or another thin material. Then the glass will sit on a flatter more well supported surface. 
Bend the aluminum bed until it is flat.
Add a 5th leveling point under the middle of the bed. You could either make it adjustable which would be tricky to get to or create a support piece accurately or get one close and shim it. 


The shimming process is probably the easiest and the one I may end up doing. But whether you shim it or do the other trick you need to measure for points that are out of flat. 

If you get a decent straightedge and some feeler gauges and/or or shine a bright light from the other side and look for spots of light between the straightedge and the aluminum bed you can see where you need to shim or otherwise adjust it. You’ll need to move it at various angles through the center and other spots on the bed. That way you can see whether it is just the middle or if the edges are an issue too. 

Also, once you check the aluminum bed, check the glass as it may not be flat either.

There was a YouTube video on the cr-10 I think that showed part of the shimming process and checking for level. I’ll try to see if I can find it to add a link here. 

I still didn't find the video I was looking for, but here is a useful related one that talks about tramming the bed (what 3d printing people call leveling)
https://www.youtube.com/watch?v=CcAmZqb-ZEE

And here's a reddit thread about the issue. Someone incorrectly says the glass plate can't flex that much but it certainly can. We're talking small tolerances. Even granite slabs flex small amounts.
https://www.reddit.com/r/CR10/comments/7d7gyh/aluminum_bed_warped_cant_get_it_to_do_anything/