The connections in the slot may be worn from age or even more likely covered with a thin coating of residue or dust blocking full contact. If you have not already done so, clean a single card (contact metal) with alcohol, leave it "lightly" wet and work it in and out of the slot, periodically wiping it when withdrawn. If available, you can also use canned compressed air to blow out the slot.
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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