Touch ID not working properly after screen replacement

I have been repairing phone for several years and always use the original home button but I recently replaced a screen on an iPhone 6s for a customer. I tested everything and it all seemed fine. The customer emailed me the next day to tell me that Touch ID wasn’t working. I have checked all the connections and tried several screens but it still isn’t working. I know that usually if the button isn’t working properly it says ‘unable to active Touch ID on this device’ but this isn’t here. I tried to register a new finger print but it keeps saying ‘Keep finger in home button until told to lift’ even if you keep your finger there. I am totally baffled and have a very unhappy customer. Any suggestions would be much appreciated.