First, clean the metal contacts on the lens and body mount with a soft cloth and a little alcohol may help, but, unfortunately, a more likely cause may be misalignment of the lens contacts due to the weight of the larger lenses (I have done this to one of mine just by grabbing it by the lens a few too many times). This issue may require professional service to one or both of the lens mounts.
Please keep in mind that my diagnosis & solutions provided are directly dependent on the accuracy of your description of the problem. As with any "do it yourself" fixes, success is a "team effort", since I can't see or touch the camera, and relies on the customer's manual dexterity and ability to follow the instructions well.
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