it takes about an hour or so for the mat to fully heat up and if you make a change it could take about an hour to show up. Did you give it enough time? Also, is the pad inside the tank or under the bottom of the tank? Are you measuring the mat temp on the inside of the tank on the glass directly above it?
I have 2 mats and 2 rheostats. I keep one all the way down at the lowest setting and it runs about 80 degrees on the glass. With the Substrate Liner installed the temp on the substrate is about 74 - 76. The other mat I crank up about 1/4 turn from the lowest setting and it measures about 91 directly on the glass. Once the substrate liner is in place it measures about 84 - 87.
But I make the adjustments in very small amounts, and tape the knobs down so they can't be moved, and monitor the temps for several hours to make sure it is correct.
As your house heats and cools, those temps may be affected. A few degrees one way or the other won't hurt. That's how rheostats work. If you invest in Thermostats they can get your temps more precise but they also cost more money.