We have investigated a response function of high resistivity X-ray CCD detectors used for imaging and spectroscopy in 0.2-10 keV energy band. Channel stop regions occupy a noticeable fraction of the device pixel and can seriously distort the shape of the response, especially for the so-called horizontally split events which produce charge in two adjacent pixels. We found that charge clouds formed inside the doped p+ region of the channel stop can suffer a serious charge loss. To investigate the details of the charge loss distribution inside the pixel the "mesh technique" was used. We have discovered that charge loss is entirely suppressed under one of the 3 gates comprising the pixel. Voltage and temperature dependences of the losses were examined, and were shown to have a dramatic effect on the mechanism of the loss and the shape of the response function. The discovery of the supression of charge loss at low voltages can have a significant effect on the design of backside illuminated devices.