First, make sure the Herpstat is set to run 100% power to the DHP (i.e. make sure 'Maxpwr' is set at 100%, and that there isn't some long ramping period that you're riding out). Make sure also that the thermostat doesn't think it has reached target temp (i.e. make sure that the power output remains at 100%). Also, if the Herpstat doesn't reach target temp in some reasonable amount of time, it (if set to do so) cuts power to the heat element in case there's some hardware fault.
I don't use DHPs, but: if you have a heat element that is being powered but still not attaining the target temp, then that heat element is underpowered for the conditions.
If you describe the enclosure size and construction, distance to basking spot, and room temp, someone might be able to confirm that's an undersized DHP and recommend what size DHP you need. Alternately, just get a really powerful one and let the thermostat do its job.
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