Inking is almost its own artform. Its not really like drawing, so much as identifying the light and dark 'shapes' and lines and using texture as well as lines to convey shadow creating a sense of space, weight, and depth. Seriously, a good inker is worth their weight in gold. But if you want to learn the skill, you have to really look at it as a seperate technique. Drawing is not inking, inking is not drawing. Part of the reasons why inks look 'hollow' is that there is insufficient attention paid to the lights and darks, completing lines, and adding line weights and textures.
You might want to take some time to practice inking on drawings that are not your own with some reference (video or book, I suggest "The Art of Comic-Book Inking" by Gary Martin.) If you work on art that isn't yours, you tend to be more attentive to detail of learning the skill you are practicing rather than focusing on redrawing the sketch under it. I dunno if it makes much sense, but I've personally found working on other people's art makes me focus more on learning the skill I'm practicing, rather than falling back into a rut of old habits. When you come back to work on your own stuff, you'll bring those new techniques back with you.
As for programs...
I'm guessing you are using OpenCanvas 1.2ish, NOT OpenCanvas 4.5 (
http://www.portalgraphics.net/en/), [portalgraphics.net] which is a slightly different beast and far superior IMO for drawing and inking. The ink tool is vastly improved in OpenCanvas 4.5. Photoshop, in general, has always been a rather poor choice for inking unless you use the vector tools and block ink. Which is rather tedious. If you want to get into vector inking, for the quick and dirty, Adobe Illustrator CS3's Livetrace function is godlike for turning line work into vectored inks. Truly awesome and timesaving. Some people also like to use Flash for this.
Manga Studio, although its touted for 'manga' can be useful for any comic style and has some very sweet inking features, including tapering, pressure sensitivity, and auto smoothing for lines. I highly suggest giving it a whirl for inking specifically. If you do a little hunting about, you can still find the 3.0 demo.
If you need a freebee, Pixia isn't too bad. I've played with it, but it doesn't have the clean crisp I like that I can get out of Manga Studio or Opencanvas 4.5.
If you are willing to spend money, one of the uber pressure sensitive apps is Corel Paint X. Seriously, for real art medium like quality, you can't really beat this program. One of the most responsive programs I've played with, although I personally like pen and paper the best ( I mean the real deal, not digital equivalent), its been a mainstay of digital artists for years.
I'd also suggest that you make sure your tablet drivers are current.
I don't know if this applies, but I have also found there to be quite a difference in tablets from a few years ago to today. Also, not all tablets support tilt sensitivity, and only pressure. Tilt is supported on the Wacom Intuos and up, but I don't know about your tablet. Most low end tablets also only support 512 levels of sensitivity, whereas higher quality tablets support literally double that. Going from a lower quality tablet to a high quality one made a HUGE difference in my art comfort and quality.
Its sort of the age old problem of your materials. The better the tools, the better the product.