I'd recommend first making sure you have the latest version of ArtRage on your TabletPC. The current version is 2.5.20. You can check your version by going to Help --> About ArtRage, you should see the version number in small text. Also you can select 'Check for updates online in the Help menu.
If you don't have the latest, and ArtRage came with your Motion TabletPC, contact Motion to get the update. If you purchased ArtRage separately, you can download the update via our Member Area at http://www2.ambientdesign.com/members
1/ Click 'Register' and follow the instructions
2/ Log in and go to 'My Products'
3/ Add your registration key
This problem may relate to Ink Services. Once you have the latest version, you can disable the use of Ink Services in ArtRage by placing a file called 'no_ink.inf' in ArtRage's resources folder
c:\program files(x86)\Ambient Design\ArtRage 2\Resources\ <-- File goes here
It doesn't matter what type of file you use, just the name. Once you've done this, start up ArtRage again and see if you have pressure again.
I hope that helps!