@Scarlett please see here No se puede mandar mensajes a los contactos
there it has been discussed even more. It seems that it is random. For me it first worked after the October update. Then like in your case did not work at all, not for a single Zoi contact. Then after weeks it worked again for all Zois, then did not work and now it works for some Zois but not all.
Really unclear what is causing this and why it is changing.