I believe zoo med suggest 6 to 8 hours or overnight for substrate to reach temp from what I vaguely remember. I had a 50 gallon Exo Terra tank with a 24 watt UTH set to 90* My substrate would be around 88* but it took it thru out the night to reach it. So you might give it some time and periodically check on it. Also check the depthness of your substrate could be to deep. But don’t go above 92* so if your snake burrows it has a less chance of cooking itself.