nozzle not heating

[roshan_ultimaker 0]

currently my all ultimaker 3d printers are working well but I am facing an issue in my other printer having 1.75mm filament diameter.problem is after starting print nozzle temperature reduces drastically automatically.bed temperature increases above 150 degree.due to improper nozzle temperature drop material stops extruding.can you tell how to resolve nozzle temperature droppage issue?

[GregValiant 1,330]

What make and model is that 1.75 printer?  If it is other than an Ultimaker printer then can you copy and paste the StartUp Gcode here or post one of the problem Gcode files?  How are you getting the gcode to the printer (SD card, Octoprint, USB)?