It's easy to explain, I hope. You see, what I think happened here is the Harry that had been attacked by the Dementors was very confused and exhausted (proof: unable to make patronus) and because the Dementors were already on him and Sirius, he couldn't think of a happy enough memory; translated: he was too confused and was unable to concentrate on anything but that he had his Godfather. The Harry that went back in time yelled, yes, but the Harry of the past didn't hear him and they didn't put it in the film because they were showing that part with past-Harry's point of view. I hope that's good, ha ha. If that's not it, maybe the film-makers didn't realize it. Or they did and thought that the past-Harry would realize it wasn't his father (book).
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