I'm assuming this is a tablet pc and it's brand new? If you search for "Lenovo x220" or just "Lenovo and SAI Paint pressure problems" does anything come up? If nothing does I'd try to return it or start thinking about giving up SAI.
I had a Fujitsu tablet pc and I just could not get SAI to work with it. There were no reports or solutions to the problem online so I had to give SAI up while I owned it. Till this day I don't know what exactly was wrong with it. I suspect it has to do with SAI though so it makes it even less fixable.
Maybe somebody has experienced the same problem with a Lenovo but they've fixed it? I'd try searching the forums maybe.
It's weird because I've seen youtube videos (well like two) of someone with my tablet pc having sensitivity work in SAI. I did what they said and downloaded the update but I still can't seem to get it working. But I think it's possible.
Well I have some last suggestions in order from easiest to hardest.
1. Look into Sai's setting, the pen pressure might be off or something like that. I haven't gone into the options in a long time, but the other day I heard there's a pen and a mouse mode. 2. Search for SAI pressure sensitivity problems, but do not add lenovo since we know it should work. 3. I don't know which drivers you tried installing. Try wacom's drivers but also try Thinkpad's wacom drivers (go to their site, go to the downloads/drivers, find your model, etc.) Sometimes they're different from wacom's more generic drivers. If neither work, try older version of the drivers if they're available. 4. Call Lenovo (Wacom will not be able to help. Lenovo just uses their hardware, and they set up everything, often with, like I said, custom drivers).
____ Harder Stuff, not suggested you try unless you know what you're doing... This is just what I attempted when that tablet of mine didn't work.___ 4. Reinstall SAI. I'm not sure if it retains the activation or whatever from when you bought it or how hard it is to get back. 5. Install older versions of SAI. 4. Reinstall Windows. If you've never done this though... don't even think of trying. I've done this 10+ times. The first 2 went fine, 3rd I erased someone's hdd, and some later ones messed up minor features that I never got back.