Although this is a rare occurrence, it is possible for the contact form to reject a constituent who lives very close to the border between two districts (oftentimes from a split zip code that is shared between different Members), even if this person is contacting the correct Member of Congress.
Why does this occur? We use the house.gov official "write your rep" form to determine if an address is in a district. Because there are hundreds of millions of addresses, there are just too many of them to ensure 100% reliability. It's rare, but it does happen.
What can you do to fix the problem? If it is just one constituent that is experiencing the problem, you will likely receive a phone call from the person, and after gathering the appropriate contact information, you can simply enter said information manually into the system using the Add a Person feature.
If you find that a particular split zip is giving you a lot of trouble (multiple occurrences), one of your office's Account Owners can choose to accept all mail from that zip code by adding the zip code to the "Constituent zip codes" section under Settings>Contact Form Settings. If you have multiple zip codes to enter, place only one on each line. Or, if you would prefer us to do it, just send us an email at email@example.com.
Finally, you should also submit an error report to the official record keepers so they can fix those addresses (use the last option under the "Type of Comment" field).