If you are having trouble getting your heat plate to reach temp, try checking the resistance with a volt meter.
First, set your volt meter to measure resistance/ohms.
Next, place the voltage meter pins on the wire leads coming from the heat plate, matching red-red / black-black. You are looking for a resistance under 2 ohms. If the resistance is greater than 2, you will have trouble reaching heat plate temps higher than 75C.
*Check the Printrbot Heat Plate product page for your heat plate specifications, i.e; max temperature.