How to enable 'Send Interview Invite' to candidate who applied to a Job requisition

Summary

How to enable 'Send Interview Invite' to candidate who applied to a Job requisition

Content

Hi,

I went into the candidate profile of a candidate who applied to a Job requisition. Among the available actions, there is the possibility to send interview invite to the candidate, in order to send a notification to the candidate, with a link which allows him/her to schedule an interview (phone, in person, Web conference). This functionality worked very well , but suddenly It stopped working.  Does anyone have any suggestions? 

Thanks a lot,

Alberto.