No service - antenna problem

Hi.

I work at a iPhone repair shop and now have a problem I have not really encountered before.

The customer was in earlier to repair the power flex, which we did. A couple of days after the customer came back saying that the phone constantly showed "No Service". The time and date was not set right, so I did that and still it showed "No service". i tried with another SIM-card to see if it was the card itself, which it was not.

Then I changed the dock-connector to switch the antenna. This had no effect either.

After getting a bit frustrated I changed the whole frame to lock out any factors. This did not help either. I then changed back to the original frame and this time there was slightly reception, like one bar. And quickly it dropped to "no signal".

Do you have any tips? Something I might have overlooked? I cant really see any damage on the logic board and I cant really understand how that would´ve happened as I have dismantled and reassembled 100s of iPhones while I have worked here.