I dont know why I suddenly came into this when I started hovering into the 1500 trophy range when I got to TH8, but there are 2 people on my revenge list that I cannot get revenge on due to the fact they are always online or always have a shield active.
Arash Kamanhir attacked me 7 days ago
Clan is in a foreign language I cant read
'' (yes his/her name is 2 apostrophes) attacked me 2 days ago
THAILAND ONLY clan
I understand running a farming base will generally give you a 12 hour shield since most people will "snipe" the easy Townhall, but the odds of them always being online when I am or always having a shield when ever I hit the revenge button is extremely slim. Im usually on as often as can be when trying to farm resources but these guys are never offline unless they have a shield.
Valid online would be the player, or multiple players keeping the account online for 6 hour blocks. However, if you were persistent for 6 hours of trying you should find he gets kicked off for 10 minutes in which you have a small window to attack that he cannot avoid.
More likely though, is that this player is using a bot program to keep his account "online" all the time, most likely through an emulator.
Botting usually is the case when you see them "online" at all times of the day. Shields on the other hand just mean they lucked out on getting usually a free shield.