Maybe it's because of the plentiful jokes of Iku failing to do so?
Jokes aside, while she can read the atmosphere just fine, canonically, she lacks understanding on how to USE that knowledge to her advantage. Which often leads to the same situations as not being able to read it at all.
Atmosphere...I want to marry Older Brother...Electrostatic induction...Eeek!(Converted into human years)ZzapYou must read it.When I grow up...NnnnhLittle Iku, Age 8