Now, there has been a lot of problems over this I'm gonna tell you what to do with some handy screenshots.
It gets really annoying when you loose pressure sensitivity and you're in the middle of drawing and that is because sometimes SAI can lose you're settings, after you have been told to remove all preferences and that, you, like me, may get no response from your pen but only when you press hard on it. I have found a great solution.
This might not work for all users, because of the type of laptop and the type of tablet because it has no sensitivity installed on it at all., but I hope that this will work for you.
Note: Before we go on I forgot to say that you need a tablet, Wacom preferably and you also have to have purchased SAI.
So, thats what it looks like. yours might be different though, but the next steps will help. Go to Others - Options - DigitalSupport
Your settings should be at 0%, if it is at 100% then that is why you have to press so hard, if it is other than 0% then set it to that.
Before you fiddle about with SAI, you still mustn't, for smoother lines go to Advanced settings and change your Quality to 4 (smoothest)
Size and Dens ticked. Come out of sai (if in the middle of project save as a .sai)
Close then re open SAI and feel free to draw!
Any questions ask and I'd love to know that I've helped you ^^
This will also work with trials, there are no more possible ways to download SAI so you must buy it ^_=