Hey there. Iron Banner does not have a streak bonus though the Valor you earn at the same time, will do. You have to wear IB gear, equip an IB emblem, and complete challenges, to increase the reputation boost.
Note that the challenge looks at the highest level of the challenge completed on any one character. So playing 3x games on 3 characters to get each to their first level of the challenge, will give a '1 out of 4' boost. But playing 7x games instead on one character to complete their second level, will give a '2 out of 4' boost for further games. (Unfortunately the bonus for '4 out of 4' is bugged so get one character to 3 first and you'll be at the maximum challenge boost for all further games).
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Appreciate the feedback, I'll give it a go and update. Hopefully I'm just ignorant and this is the problem.