When getting a Wacom Tablet second-hand the pen pressure may not be working. It could possibly be due to faulty hardware, but before you throw it out try this.
1. Install the proper Driver.
There are specific product drivers that enable the pen pressure. Check out https://www.wacom.com/en-ca/support/product-support/drivers to find your tablet and install the driver.
This was the problem in my case and now I can do the digital painting!
2. Replace the Nib
Just like a pencil, although not as frequently, the nib does go through wear and tear. You can replace them. Go to https://estore.wacom.com/en-US/accessories/accessories-nibsc.html