Some agent don't appear in "To:" field selection list

Some agent don't appear in "To:" field selection list

I have a problem when adding an Agent's name to the To: field of an email.

For some agents, you can start typing their name and then select the person from a drop-down list. (See attachment for example.)

For other agents, you have to type their complete email address.

Does anyone know a solution to this issue?

Thanks,
Tim