No discovery tool can guarantee it finds all subdomains.
The only way to be sure that you have discovered all subdomains is to have access to the DNS server of that domain and look at the zone file. We know that this is not always possible.
You should be aware that all subdomain discovery tools work as best-effort and none can guarantee that they have found all subdomains. However, our Subdomain Scanner is one of the best penetration testing tools out there.