not letting me use my activation key

[Niwashi]

I have 2 service accounts because a while back ago I picked up havensward twice. So I have had a spare account.

By any chance, was one of the two bought through Steam and the other not? That could account for a code working on one and not the other, since Steam and non-Steam codes can't be mixed on the same account.

If the game was purchased from Steam, then all expansions have to be purchased through Steam, and if the game was not purchased through Steam, then expansions cannot be purchased from Steam. (It seems like a pretty dumb rule to me.)