When trying to develop monster topspin shots off your ground strokes, it is important to visualize a whip. A whip is thicker at the handle and tapers down towards the tip. When snapped, it can create speeds surpassing the sound barrier (cracking sound). Your body is similar to a whip in design: your trunk/torso is generally thicker and moves slower. However, your shoulder, biceps and forearms are considerably smaller and, therefore, move faster.
Once you start uncoiling into a shot - particularly the forehand - the momentum generated travels through your arm towards the racket. Here is where things get interesting: although your arm moves in one direction following its natural path, the wrist often snaps (or, some say, forearm rotates) in order to generate topspin (a deviation from the normal angular momentum of the arm). This "snap" is a slight (for some, not so slight) brushing of the ball which creates the spin. However, to do so, the wrist must deviate somewhat from the natural path of the arm (obviously, it still remains attached to it) while holding on to an object (racket) about half its weight (human hand weighs between 22 and 30 ounces).
The issue that we're dealing with is control of the racket. In order to have a good topspin it is important to have a strong grip (although not choking the racket) as well as a strong forearm in order to commence the topspin motion. Remember that momentum = Mass times Velocity. In this regard, although the hand (tip of the whip) weighs less than your body, it's also moving faster thereby creating momentum in a certain direction. To "break away" from its natural path, a greater "force" is necessary (i.e., forearm and hand strength). Tennis players, like baseball players, have understood the importance of hand and forearm strength for decades. Some have squeezed broken tennis balls as a way to cross-train; others have lugged around forearm builders such as clamps and pulleys.
One of the best way to develop forearm-grip strength for tennis is to practice a lot of drop-feeds where you are isolating this portion of your body. Try to LIFT the ball 6 feet over the net by BRUSHING the ball (rather than scooping it). Once you get the concept, you can transition to slightly more sophisticated workouts such as hitting against rapid-fire balls on the ball machine (minimum of 300) or hitting 1000 volleys in a row against the backboard. The key is to "feel the burn" in your forearm and develop grip and forearm strength. This concept is particularly important for female players. Unlike guys, girls tend to not be gym rats and they don't often play sports which require arm strength (e.g., baseball or football). Therefore, although girls generate a great deal of force with their torsos, shoulders and arms, they tend to fall just short of mastering the topspin due to their relatively low forearm strength. As a result, a lot of female player tend to have flatter shots and a lot of teaching pros perpetuate this mistake by teaching the shots that they see on TV rather than addressing the physical issue.
Nevertheless, things are starting to change and some players have broken away from the general mentality. Take, for example, Samantha Stosur. When she first started on tour, she was a very good player but her shots lacked the "bite" necessary to penetrate the court. Take a look at the size and definition of the arm, particularly the forearm (note: size is only one indicator of strength but it's not exclusive).
Now take a look at the picture on the right taken in 2011. Forget about the racket finish - we are only using this picture to point out the definition of the arm, specifically the forearm. If you watch Samantha play now, you will notice that she plays similar to a male tennis players: not just powerful shots (Venus and Serena have been doing that for years), but shots with heavy topspin.
Given the pace of the game, these shots would not be possible without forearm and grip strength. Therefore, rather than making excuses as to why you're not hitting decent topspin, take the initiative and start improving yourself physically and then see how it goes from there. With modern technology (lighter rackets; more powerful strings; awesome gym equipment), there's absolutely no reason why you should not be able to generate monster topspin groundstrokes.
Of course, don't forget about the other components of the shot (e.g. footwork, balance, torso, preparation, etc.) and to practice, practice, practice. Before you attempt to tear phone books and crush rocks (and destroy fluidity and range of motion in the process), know that this is only a small piece of the puzzle (Stosur's as well as yours). However, when encountering obstacles in the development of these shots, keep in mind the strength ratio between your body/arm and forearm/wrist.