Pressure-sensitivity in Sketchbook Pro on desktop

For desktop users, if you are having issues with pressure-sensitivity in Sketchbook Pro, there are a couple things you can do. 

1. On Windows, make sure Windows Ink is enabled in your tablet or pen driver. Sketchbook Pro will not have pressure sensitivity with Windows Ink disabled
2. Ensure you have the most up-to-date tablet driver on your display tablet or pen tablet device. If you just did an OS update and lost pressure sensitivity in Sketchbook Pro, double-check your tablet manufacturer's website to see if newer drivers are available
3. Re-calibrate your stylus‍ 
4. In the Sketchbook Pro menu bar, select Edit > Stylus Responsiveness and try making adjustments.

For info on stylus tilt, check this article Setting stylus tilt‍. 

Which pressure-sensitive stylus does Sketchbook support on Android

For Android users, currently, only S-pen capable devices (Samsung) support pen pressure on Android.

I used to have pressure sensitivity on my iPhone, but now it's gone

Apple had a feature called 3D Touch that was available on iPhone 6S through iPhone XR. 3D Touch enabled pressure sensitivity when drawing with touch in Sketchbook on those devices. With the iPhone 11, 3D Touch was discontinued in favor of Haptic Touch. So this kind of pressure sensitivity is not possible in Sketchbook on newer iPhones.