Same here. My tablet used its sensitivity by default, and if it's functioning properly, that's what it should be doing.
Are you sure it's not a problem with the tablet itself, and not just its model or something? I'd say you should try using its sensitivity in another program, just to test it out (Photoshop, Illustrator, Paint Shop Pro). Some programs don't use sensitivity, and others (like PSP) only use it for some tools, so make sure the program and tool you use to test it work with it as well.
Also, it may be that your tablet is getting old, and maybe you need a new one. If you really need help though (like, if it works with other programs or something along those lines), you may want to contact the manufacturer with the problem and see what they say.