Could be several:

 1. They are unsubscribed (then you would maybe see that the contacts entered the email activity)
 2. They do not have an email in contact/allsubscriber (but your journey configuration wants to grab that value from the all subscribers - then the email value would have been chosen from a dataextension that is linked to the contact model)
 3. These Contacts do not have an email in the dataextension that is used for sending if your journey configuration says that the email address should be taken from the journey data
 4. They are marked for deletion
 5. They are suppressed in any other way
 6. The email is somehow bad configured that it just worked before but
    not afterwards
 7. Exclusion Scripts of the Email may have kicked them out (then you would maybe see that the contacts entered the email activity)
 8. Supressed Audience could kick the Contacts out
 9. AMPScript in the Newsletter could have raised the RaiseError
    function (then you would maybe see that the contacts entered the email activity)
 10. Any other activity that may be in front of the email cannot process
    the contacts or kicks them out (because we do not see the journey, i
    have to make an assumption).
 11. ...

Here some stuff to help yourself:

 1. Open JourneyBuilder Tab and Go to History (tab on the top of the
    screen).
 2. Select the specific journey on the rigth side  
 3. Select some activities you want to monitor and/or a specific date range.

Watch the contact keys and tell us more about the informations that are presented inside the history. This makes it a lot easier to find out the cause and a screenshot which shows the activities of the journey might help aswell (just make sure to obfuscate parts you don't want others to see)