If the tablet responds to touch and movement but not pressure, this typically means a problem with the WinTab service. Tablets come in two main varieties: WinTab and N-Trig. They are totally different protocols and incompatible with each other. ZBrush supports WinTab since that is the dominant protocol in the industry.
Microsoft owns N-Trig, which means that all Surface machines use that protocol. I do not know which one Huion tablets use.
If Huion uses WinTab and have installed the most current drivers from the Huion site, then it’s likely that simply restarting the computer will fix the issue. If not, then you may need to uninstall the tablet drivers, downloading and installing a fresh copy from Huion.
If Huion uses N-Trig, check with them to see if they have a WinTab driver available. Otherwise, you may be able to find a WinTab driver from Microsoft directly. They have released supplemental software packages for the Surface machines that include WinTab. If it is not possible to get WinTab running, your tablet will simply not have pressure sensitivity with ZBrush. Pixologic cannot implement N-Trig support without sacrificing WinTab support.