I've been having this problem with my delta 3D printer where my nozzle isn't moving flat with my print bed (as if the bed is bent) but I have used the edge of my steel ruler and it seems to be perfectly flat. I have properly leveled my bed with a piece of paper and have checked the whole printer to make sure it's square but the problem continues.
What else could be the problem?
Addition info (from comments)
The gap grows and shrinks in a parabolic manner and it makes it impossible to get a good first layer. the printer is a FLsun Delta Kossel. The links don't appear to be loose. Is there a way I could share a video?
Upon further inspection I found a bit of play in one of the links, I tightened the bolt and the play is gone but it didn't fix the problem.
I have been playing around a bit and I found that the nozzle is closer to the bed in the center than it is near the edge.
I did some research and it's a calibration issue but I have no idea how to fix it. Does anyone know a quick and easy way to calibrate a Kossel Delta 3D printer?