Hello @RichardG286 ,
For the sake of troubleshooting, can you confirm that you're setting the search parameter for Email Domain, and not leaving it in the default Email or Name? Additionally, when typing out the domain, are you adding the Top-Level Domain (e.g. .com, .org, .net)? The TLD needs to be included when searching for the domain, otherwise results will always bounce back. For example searching for just "gmail" under Email Domain parameters won't show any results, but searching "@gmail.com" or "gmail.com" under Email Domain parameters will show results.
If you've confirmed that your search parameters are for Email Domain, and that you're including the TLD in the search, and that the spelling is 100% correct, then we'll want some specific examples. Please reply directly to the @ mention email you receive from this response, and include specific examples of domains that you know are in your contacts but aren't returning results when searched.