My facebook messenger webhook isn't receiving messages for the pages which are not created using the same account as fb application

Question

Webhooks sends message for:
The pages created in the same account as app account.
I can subscribe the pages for the webhook using page-id/subscribed_apps and it starts showing up in Webhooks Subscribed pages: XXXXXXXXX

When subscribe the pages for the webhook using page-id/subscribed_apps for other user account pages it returns

{
"success": true
} 

but it never shows up in the webhooks subscribed pages section of app.
It doesn't even sends message to webhook for those pages.