I haven't used this for a time but as I had already discussed sending an invite to someone as a Guest (they have their own small Tree on Ancestry), decided to do so to allow me to check it worked. I didn't have their username to hand so chose the email option - the person is a known to me anyway- and can report that went through and quite quickly as a matter of fact. Perhaps it is the username option that causes the problem although I cannot see why, other than an out of date email lodged with Ancestry?