I went through this exact phase about four years ago. The general problem in 99% of these cases is the fact that you're thinking too hard about your wrist position and putting much too much into the rotation of the wrist when you bowl. Here comes another coaching section excerpt!
A wrist spinner's basic grip is to hold the ball, with its seam horizontal, cupped in the first three fingers of their bowling hand, with the index and ring fingers roughly on opposite sides, and the middle finger fractionally closer to the index finger – resulting in all three fingers lying across the seam. It's perfectly normal for the thumb or the little finger to rest on the ball, but rare for either to do any actual work when it comes to spinning the ball. The hand action that imparts the spin itself is similar to that of the finger-spinner – the “turning a doorknob” motion – but since it's in the opposite direction (anti-clockwise for the right-handed leg-spinner, and clockwise for the left-handed chinaman bowler) and as such involves the wrist to a much greater extent, it's much more forceful. With practice, when the ball is spun the seam will remain in approximately the same position – resulting in achieving more grip and hence more turn as it lands.
Returning to the ever-trusty tennis ball, if you take it in the wrist-spinner's grip and simply lob it gently, underarm, into the air – remembering to include the hand action as it's released, you'll see it spin a significant amount either to your left (if you're right handed) or to your right (if you're a left-hander). A wrist-spinner's full action is generally closer to “front-on” than “side-on” – the bowler looks in front of his front arm (or between his two arms as they're raised) and the “imaginary lines” angle across the pitch in the direction of the covers and midwicket. As with finger-spin, it's important to “pivot” strongly on your front foot as you prepare to release the ball, and equally after your release to “drive” or follow through strongly with your back leg (the same side as your bowling arm). The bowling arm is generally vertical, or near-vertical, which allows the bowler to gain as much bounce as possible and get the ball above the batsman's eye-line more easily. It's not uncommon, however, for wrist-spinners to bowl with much flatter arms as either a natural action or simply as a variation.
The practice with the underarm action is a very good way of getting the rotations in the right direction. To recap, start with your palm upwards and rotate it anti-clockwise, finishing with the palm downwards, so the ball spins to your left after it bounces. You can then incorporate this into your action.
I've written a lot more on variations, etc, but I'll post that once you've got the basics sorted. Feel free to add me to your MSN list (address in sig...)