Usually this problem seems to be a problem with the pen, but can also be a problem with the driver, and sometimes can be the tablet itself.
Try the Common drawing troubleshooting steps these should resolve things most of the time. If that doesn't work, contact support.
Some things to check
Does the pressure work in the tablet driver pressure test region?
What happens with the Krita tablet tester?
Krita > Settings > Configure Krita > Tablet settings > Open Tablet Tester
See what happens with Windows PNP drivers instead of the manufacturer drivers: Testing with Windows PNP drawing tablet drivers
Some other things to try
Reset the pressure curve in the tablet driver
Reset the pressure curve in your app
Reset the pressure curve in the brush in the app you are using
If you recently swapped nibs, see if going back to the old nib fixes the problem
Links