Ok, that could be your problem.
Gauge thermometers really have no way of measuring the basking SURFACE, they can only measure the air around the gauge.
You need to either get a temp gun, or a digital thermometer with a probe. Once you get one, you place the probe on the basking SURFACE and see what it reads.
It sounds like your basking spot isn't hot enough, which is why you really need to find out what it is to make the adjustments. Temperature is not something you want to guess at with beardies; since those gauges can be up to 20 degrees off, you basically are guess with them.
-Brandon