If you are using macOS visit the guide here.
In Illustrator the brush you are using must be set up to use pressure, some brushes do not have this turned on by default. The working space will need to be set to “Painting” to easily find the pen settings to check or change this setting.
- Click on the top right icon and change the interface by "Painting" as seen on the screenshot below.
- Now that the workspace is in Painting mode, all the brush settings are shown. Select the brush you wish to use.
- You can also create a new calligraphic brush by clicking the "+" button. If you do, skip to step 4.
- Once a brush is selected select the menu button at the top right side of the brush window and select "brush option"
- Once the brush options menu opens you can find the three data points: Angle; Roundness and Size. By default, all three are set to "Fixed" as value which means there is no variation. To use pen pressure, change the setting for “Size” to "Pressure.”
- Next set the variation range. The more points attributed the more the pressure variation will be when drawing. We recommend at least a variation 5 point to start.
- Select OK to save the changes and test the brush.