Yup, either missing or loose screw on this part in particular:
iPhone 4 Rear Camera Replacement
At step 7 the green circled screw provides the antenna contact from the housing to the board. If it has been repaired before the metal clip on the metal cover itself is sometimes missing meaning either the metal cover will need to be replaced with one that has one or you would have to solder a short wire to create the contact.
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