Clear aligners, like any removable retainer can make limited changes to your teeth. Closing small spaces and turning a front tooth and examples of what they can do well.
Like all removable appliances, they only work effectively if worn all the time. Teeth do not know what type of appliance is pushing on them. It takes 11 hours of continuous wear to activate the bone moving cells of your body. The teeth and bone respond to light, continuous pressure.
Clear aligners are not attached to the tooth as well as a traditional brace. So, they have trouble moving teeth that are more cylinder shaped (eye teeth) and more trouble moving teeth vertically. Often the dentist will add acrylic handles (brackets similar to a traditional brace) to the teeth so the clear aligner will have a better grip on each tooth. These acrylic handles will be removed after the treatment is completed.
If you have short teeth or swollen gums, this makes it more difficult for the clear aligners to grip your teeth. If the clear aligners do not seem to be fitting properly and snugly to the teeth, they are not working properly and need attention from the dentist.