I can't link parent sponsor account to child student finance account

My son has set up his student finance account and triggered the email for me (as his parent) to link my account. I have an older child which I've already sponsored for student finance. However, even though I've correctly input his reference number and date of birth it's showing the error message:
"We can't find your application right now
Your personal details do not match what the student provided us with in their application. Check if you are using the correct account."
_{What can I do to link his account to mine?}