Jump start heat mat not heating up
 

So I set up my matt today and put it directly on the window sill which I believe to be made of fake marble.
My dilemma is it doesn't seem to be working. When I
touch the pad it doesn't feel warm. Is that normal?

assuming you have it plugged in properly and the right way up, I would return it. I had a similar problem and I got an exchange from Amazon at the time.....

Agree but try another outlet first.

You should be able to feel it, but it is a subtle amount of heat. Is their a little switch on the cord you need to turn on? If not and it doesn't work on another outlet, I guess you need to return it.

If when you touch the pad it doesn't feel warm, for testing are minimum 2 methods; measuring temperature with thermometer (after few hours of POWER ON) or measuring resistivity with ohmmeter.
The heating mats are designed to produce a maximum soil temperature of 35ºC (95F).

I've sold about 150 of those over the last few years, and actually, the Jump Start (Hydrofarm) mats do not have a specified maximum, but are rated as having the power to boost conditions about 10°-15°F above the ambient temperature.

They are just resistance heating elements, so radiate and conduct in all directions, so - assuming it really is working correctly - sitting directly on a "stone" slab might be sucking all of the heat out of it.

You might consider putting some sort of insulation between the window sill and the mat - even a towel will do.

Thanks Ray! What else besides a towel can be considered
a insulating element? I am going to try a towel to see if
that helps but I am concerned about the stability of the
plants on it.

A strip of thin foam insulation, cut to fit the heat mat would be fairly neat and stable.

I realized after a few hours that mine was on a stone sill, and just cut a piece of cardboard to fit underneath it exactly. Probably not as stable as something rubbery/foamy, but it'll do in a pinch.