Simba,
*sigh* okay so I agree with the assessment that Cuban Motion takes time. It really does, but for it to develop at all you must be able to intelligently explain to your students what exactly they are developing.
Depending on your framework, and whether or not you want your students to be any good, telling them to roll their foot, without weight isn't even proper technique. It is all that most are willing to do...but it isn't right. By the way if you are telling them not to take their feet off the floor, that isn't right either. You end up with mediocre dancers who don't know what the hell they are doing, and shlop from one movement to the other. There should be some crispness to Latin, which requires precise placement of the feet.
I'm going to give you very GOOD advice. Spend the money and get some coaching from either Linda Dean or Ron Montez, or any number of other high level professionals who can tell you what good Cuban Motion is and how to teach it to your students. Yes they are expensive and they are worth every penny.
In the meantime...Cuban Motion does not arise solely from the feet. It has much more to do with the knees, in terms of straightening and bending the knees at appropriate times, settling or not settling into the hips based on the step you are executing and maintaing proper balance. On top of that the hip movement and your lines come from the proper contractions and release of core muscles. You must also develop core strength because where your weight is over your foot determines if you are even in proper position to pivot or swivel. You can't control that weight without a strong core. You also can't control balance without a strong core.
Cuban motion also arises from proper foot placement, and that does not mean a foot roll with no weight. The only time you actually roll the foot like that is when you are going to the side and if you are doing it right, for a small moment in time as your feet collect your entire body weight will be on that rolled point of the foot. In forward motion and backwards you get a lot of the Cuban motion from rising one hip enough to allow the other foot to pass without any movement to the side (no ronde), which means inevitably the trailing foot must flip and drag the pretty little toes on the ground. As my instructor says, if there isn't blood or toenail polish in a trail behind you you didn't do it right. This is just the very basic beginnings. Every step has a different flavor and a dfferent set of expectations that go with it.
You just have to get a real coach to teach you so you can in turn teach this to your students.