It may not be that you're not lowering enough, but that you're not lowering soon enough. It's not necessarily the problem, but a definite possibility.
Remember that you should not be lowering as you step forward or backwards, so you must therefore have all of your lowering completed beforehand.
This is particularly easy to overlook when moving from one forward step to another, such as step 3 to step 4 of a Twinkle or Open Impetus. Students are often taught to maintain tone in the foot and delay lowering of the heel until the body weight passes over the foot. The problem is, if you really wait until your weight is over the foot before you begin lowering, you will be well past your foot when you complete the lowering, which is late. Therefore, you need to lower your heel and begin compressing in the knee before your body weight fully arrives over the foot, so that when you do finally arrive, the lowering and compression is complete.
I can't really think of any "exercises" that will help you learn how to lower correctly, because I don't think the actual action of bending your knees is your problem. If I told you to bend your knees right now, you could probably do it without too much trouble. More likely it's the timing of your lowering action, or the way in which you are bending your knees (butt sticking out, body pitching forward, etc). So for this reason, the best advice I can give you is to continue to work with a professional instructor on your lowering as it applies to the actual dancing.
I know it's not very romantic, but it will work.
Sincerely,
Jonathan Atkinson
