problem with the bed temperature

hello i have a little problem with my makergear prusa with marlin:
the bed don't raise the temperature over 68°C i had tried to control the alimentation (800w atx) and look all right (the tension don't lower when the bed is heating
can be a pid problem? i had tried to change the pid setting on marlin (i use the default for makergear in marlin configure.h) with the ultimaker default setting and the temperature reaches 77 °C
any advice?

Sounds like a voltage issue.

What reading are you getting to the bed?

voltage are correct 12v and dont go down when the bed it's heating... theoretically if alimentation it's low the voltage must go down i think...tomorrow i try attach the bed to an external alimentation