The fact is that iShellz's private messenger that you are probably using sends PMs to users with IDs between 5,000 and 25,000,000. I think if iShellz/Bradley updates it, there would be a slight increase in chances of the message being sent to active users (let's say an interval of [15,000,000 ; 30,000,000]).
That's usually what I do but Gaia has this new thing that if you send the exact same PM to a person then it wont send OR if you try sending the exact same pm and get the error and try to make a new one it also blocks you from sending it.