One of the reasons that a link in mail body is not rewritten by Click-Time URL Protection service is due to it being a schemeless URL.

In the initial release of Click-Time URL Protection, the service will only rewrite those URLs that are correctly formatted with a protocol scheme.

For example:

http://www.example.co.uk or https://www.example.com will be rewritten

www.example.co.uk or example.com will not be rewritten.

What is schemless URL:

A URL that does not specify protocols is called a schemeless (protocol less) URL.

Example:

//example.com/index.html

www.example.net

example.com

For more details about troubleshooting Click-Time URL Protection service, please refer the following KB.

https://support.symantec.com/en_US/article.TECH236164.html