This guide is now on Tumblr
This is a bug in many newer tablets from wacom. It only seems to affect sai.
No matter what you do from installing old drivers to messing with things in the control panel, closing excess programs for memory or reinstalling SAI your pen pressure just doesn't want to work. If this is the issue you are having, I've found the solution that works.
Step 1: Go to the folder that holds sai.exe
Step 2: Look for [misc.ini] open it in word pad (not notepad)(Rightclick + open as administrator)
Step 3: Scroll down until you find "Avoidance for bugs of Wacom tablet driver Ver6.0x later."
Step 4: Change the value I highlig