The most common cause to the problem you discribe is oxidized or dirty electrical contacts on the lens and or camera body. Most of the time using a clean lint free cloth to wipe off the electrical contacts is enough to solve the problem. You can also use DeOxit (sold at Radio Shack) to clean the oxidation. Also make sure that all of the electrical contacts on the lens project out the same distance. They are spring loaded and can get jammed in. Good Luck and Enjoy your Nikons!