Hello Karen,
The filter in the search may be forced to look for a certain type of person that doesn't currently exist. For example, if you are sending a Pre-Event Email then the attendees cannot be in the Accepted status. Likewise, if you are trying to send a Post-Registration Email, the email will look for Undecided and Visited.
To force the email you will need to click on Advanced Search and make sure the Invitee Status (or Participant Stats, Contact Group, etc) is correct.
Also, make sure the search criteria is off for the "Only include invitees who have not received this email in the last 6 months:"
Good Luck!