Everyone else going in to protect Rukia had some experience with fighting- even Hanatarou, as a Shinigami, would have had basic combat training in the Academy. Orihime has had a few fun karate lessons with Tatsuki. Ichigo had been training in various fighting styles, from karate, to raw fist fighting, to the ways of a Shinigami since he was a small child, all with the deep-set goal of protecting somebody. Chad brawled as a child, and then used his massive bulk and strength to defend others. Uryuu was tought by Souken from age 8, and his reason for carrying out the Quincy ways was, again, to protect something. Granted, he deviated from this, but that deviation made him weaker. Only when he fought Mayuri for the sake of the Quincy, his grandfather, and Orihime's safety, do we see his true strength. Going from protagonist to protagonist, the same theme crops up- the will to protect makes one stronger. When Orihime fought Kamaitachi, she had no clear aim in what she was going to defend, thus rendering Tsubaki useless. But looking again at her battle with Numb Chandelier, we see that her reason for winning- indeed, the source of her power, is the will to protect. Here, we see a strong, confident Orihime who throws down the heroic lines as good as any of the other nakama, and then delivers on them.
Now, let's compare her fights with Numb Chandelier (link 1) and Yammy (link 2). They proceed in almost the exact same fashion- a friend, Tatsuki in case one, and Chad in case two, attempts to ward off the enemy (1, 2), and initially looks successful (1, 2). Then, things start to go very wrong (1, 2). The sequences of events that follow are almost formulaic in their similarity. Orihime defends herself and the injured party with Santen Kesshun (1, 2), then proceeds to heal the one who tried to defend her (1, 2). Finally, she delivers a stunning finish with Tsubaki... At least against Numb Chandelier. So what made this attack different?
Most will say it was her lack of volition. But she shows us she has that in plenty- not only is she directly protecting both Chad and Tatsuki, but she feels that in fighting Yammy, she will be defending Ichigo, and indeed, all her friends. Next is the idea that she is weakened because she is fighiting for Ichigo, rather than for herself. I'm not at all an IchiHime shipper- indeed, I believe that 99% of the time, her feelings for him inhibit her power. This is the 1% where I feel they do just the opposite. Orihime has never fought for herself, even when it was letting her brother's vengeful Hollow take a nice chunk out of her. She's fought, like all the other siccessful protagonists, to protect others. Though he wasn't present, Orihime was taking on a monster who had just crushed the indomitable Chad so as to protect Ichigo. And again, it wasn't just him- Chad and Tatsuki were present as well, and it was also for their sake that she was willing to raise Tsubaki at Yammy. Orihime had will aplenty, and she looked just as majestic as she did against Numb Chandelier- the chapter in which her powers are revealed is even subtitled "The Majestic".
In other words, this was an instance where Orihime's will was not at fault for the failure of her attack. What she did was essentially the same as going up against a enemy with nothing but the hilt of your sword in your hand- the willpower and the courage are all there, but there's only so much one can do with a broken weapon. This is not at all to say Orihime is perfect- she has many flaws to overcome, just as all the other nakama do. However, this is one instance where we cannot say that she is to blame for not succeeding in her objective.