If you're in Vista, if there is a problem with your Windows Ink Services settings this may cause lack of pressure in ArtRage and other Ink Services applications. To fix it, create a file called 'no_ink.inf' and place it in ArtRage's resources folder ( browse to ArtRage's folder, open the resources folder and drop the file in there ). The type of file doesn't matter, copying any file into that folder, then renaming it to 'no_ink.inf' will work for this.
When you've done that, restart ArtRage. Otherwise, let me know the make and model of your tablet, your operating system version and type and I'll help further.