Why does my KMS host not activate a client in my network?

Dear all,

I am an administrator in a company and the office 2013 of one of my clients is not activating.

However, when I looked at the Event Viewer of the client, the client tried to contact the KMS host. However, nothing happened! The product is still not activated!! 

Furthermore, I manually inputted the GVLK for Office 2013 Professional Plus so that the Office 2013 of the client automatically discovers and activates against my KMS host.

However, the client's Office 2013 is still not activated!

Could you please tell me what steps should I take so that the Office 2013 is activated via KMS?

Thanks for reading

Kindest Regards