Delta printer nozzle not moving square with a perfectly level bed (as if the bed is bent... but it isn't)
Delta radius controls how the print head moves from the outside to the center.  From the comment you posted, it sounds like your radius is too large which is causing it to hit in the center and you need to make it smaller to bring the center up.  This is not always a number you can directly change as it is sometimes calculated using other offsets so you will need to find that in your firmware.  Also after doing this you're likely going to need to adjust endstops for each tower again or the Z travel to fix your 0 point.  

I would recommend following the steps listed here for your calibration.