It's been a long time since I've boosted a toon, but it "sounds" to me like a distance issue. If you have room on the third account (mage), try making a disposable level one toon to party with the rogue. Use the lock's summon to park the disposeatoon at the nearest town or safe area while leveling the rogue, keeping the disoseatoon near enough to still have the rogue get the full experience. If the disposatoon gets a bit of experience too, who cares, it'll be twenty levels or so behing you, so won't be robbing any noticeable experience.
I'd "guess" that this will solve your problem, as I've noticed a couple of times that when a partied toon gets way far away, the game acts mostly like you're not partied. Worth a try anyway. Good luck..
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