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.
http://www.symantecdomain.co.uk or https://www.symantecdomain.com will be rewritten
www.symantecdomain.co.uk or symantecdomain.com will not be rewritten.
What is schemless URL:
A URL that does not specify protocols is called a schemeless (protocol less) URL.
For more details about troubleshooting Click-Time URL Protection service, please refer the following KB.