Perks might help. Players with thousands are probably already playing when they can, but might spur some folks to log on. need to be active to get them so folks don't just log on and go to bed.
No idea what the real fix is, since I have no idea whats causing the problem. Changing player base, aging payer base, graphics, competition, cost, age of game, changing attitudes, lack of WWII interest, combination of these and more?