Some urls don't preview properly - in email templates

In setting up an email template, I have inserted a few urls for the reader to click on. In one case, despite several different attempts to insert the URL and link it to the proper website, I could NEVER get the link to activate properly in the template. HOWEVER, I sent a test message to myself and IT WORKED just fine from the email I received.
What am I doing wrong? (BTW, this same url worked properly from within several other emails and several other urls worked fine from inside this email.)