What firmware are you using? Repetier firmware uses G32 for heights computation.
On deltas such problems often occur if you go to the outside while it works in the center region. Often problems start when crossing the connection between towers. These are simple geometric errors in mechanic or wrong calibration. Calibration works by optimizing the measured points. Points outside are not seen so not considered and might even get worse.
If you can not fix this with changing some geometry variables you might want to use distortion map for this. If you only fill in corrections manually you can make it even cover the complete printer in repetier-firmware. If not only the reagion reachable by z-probe.
