It's likely that your Z homing isn't quite right or your Z probe trigger height is configured too high, so that the nozzle is a little too low when the first layer is printed. It could also be that you are Z homing and probing correctly but doing it when the bed and hot end are cold, and when they have heated up the distance between the nozzle and the bed decreases because of thermal expansion.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
