There are a couple reasons as to why one of more client tenants are not being discovered by your parent Office 365 launchpoint.
Steps to Remediate
Check if the launchpoint has already been discovered under the 'Discovered Systems' tab
Check to see if the client tenant is present in the 'Discovered' array within the parent dataprint
If it is, check the payload processing logs to see if there was an issue creating the launchpoint
Check if there's an entry for the client within the parent launchpoint's 'Contracts' array. If the client is absent in that array, we won't auto-discover a launchpoint for them (https://docs.microsoft.com/en-us/graph/api/resources/contract?view=graph-rest-1.0 )
Have the partner log into the partner center (https://docs.microsoft.com/en-us/partner-center/ ) and confirm that the partner relationship exists between their parent tenant and the client tenant