Sometimes, Photoshop stops picking up pen pressure. When that happens, the following might help.

Open the following folder (or whatever version):

C:\Users\[User]\AppData\Roaming\Adobe\Adobe Photoshop CC 2015\Adobe Photoshop CC 2015 Settings\  

If the file "PSUserConfig.txt" already exists, rename or remove it.

Start Photoshop and close it again.

Create a new file named "PSUserConfig.txt".

Open the file and insert the following text:

UseSystemStylus 0  

Start Photoshop.

That's it!