nanoDLP

Hi..Friends
I am using TOS model of 3d Printer , right now i am trying to  print calibration plate but on toggle log it says "RAMPS sync " under module and in details " Waiting for z_move_comp response from RAMPS " so it is not moving after first layer . i got this problem  several times   while testing the print or resin profiles .. what is it and how to solve it .and after force stop it starts moving 5 mm up and platform  comes down and stuck to the lower limit switch