Hello,
there are few things that can be checked. It's important that the devices are meeting all the criteria like hardware requirements, drivers, apps compatibility. ConfigMgr performs compatibility checks, so this could be one reason. Check if you have created and deployed any feature update policies that might limit which devices can receive the upgrades. On the other hand, I would also check the Monitoring logs and the Logs on the Clients. You can find any error messages which can indicate why the devices are not eligible.
Best Regards,