Did you know your skin needs oxygen to produce a tan? There are pressure points that cut off blood circulation to the skin and can cause un-tanned spots on the skin. For example, shoulder blades, calves and some times the buttock is often affected and will have areas where they do not tan. Some helpful tips next time you are in a tanning bed to avoid un-tanned spots, try flipping over half way through your session or lying on your side for a few minutes. Also try a stand up tanning machine to get a better all around tan.