Exactly what Tag said. The problem is that Poe and Phasma are supposed to be big characters, on the level of importance that Kylo and Hux have, or Rey and Finn. Phasma I could understand just being another Boba Fett who doesn't actually have any importance to the story, in regards to the OT, but that's not how she was marketed or even how she was used in the movie. We get a tense scene with her in the beginning when she tells Finn to put his helmet back on. The movie tells us she's going to be a threat of some sort. Then, when it comes time for Finn to face the stormtroopers with Luke's Saber, he fights a no name trooper. Sure, there's books that explain that trooper and give him background, but the film failed to give us that, so he's useless in that regard. Phasma could have easily been the one to fight Finn and it would have made perfect sense. Instead, she's never used except for bit roles.
To further my point on her importance, as set up by the movie, she has the rank of Captain, her armor is different from other troopers, she's the first female stormtrooper that we know of, and she's got a damn cape thing that she wears. She's made to be an important character, especially in her design. You should care that she's not used well in the movie because the whole movie is suggesting she's meant to be used.
I loved this movie and it'll probably be one of my favorites until the next two come out and perhaps take its place, but I can still acknowledge it's faults. Phasma was meant to be used more, but it's like they forgot about her in the script writing sessions and that's terrible. She did nothing that a stormtrooper couldn't do for the movie. There were no scenes of hers that a stormtrooper couldn't have been used in. Hell, the fight with Finn I mentioned, the one she should have been in, was in fact fought by a stormtrooper. She's supposed to be the female stormtrooper equivalent of Boba Fett in terms of her design and her use in the film, but for the life of me I can't see why. She's important, but never ever used effectively. That's bad.
As for Poe, he was more of a plot device than he was a character. He initiated the delivery of the map piece to the resistance, he deus ex machina saves Finn and the others at Maz's castle, and he blows up Starkiller Base because someone had to do it. The movie set up why all of this stuff had to be done by Poe, certainly, but the writing for each of these plot points were poorly done if they meant to use Poe in any substantial way as a character and not just a device. It's clear that he's meant to be an important character, much like Phasma, but he suffers the same lack of use as Phasma as well. They just sorta forgot about him. He pushes the story along because it needs to be pushed along. As a character though, he needs a lot more development.
Those are my reasons as to why they needed more screen time. Without their screen time, they don't seem as important to the plot as they should be, or at least as important as the movie suggests they're meant to be.
To further my point on her importance, as set up by the movie, she has the rank of Captain, her armor is different from other troopers, she's the first female stormtrooper that we know of, and she's got a damn cape thing that she wears. She's made to be an important character, especially in her design. You should care that she's not used well in the movie because the whole movie is suggesting she's meant to be used.
I loved this movie and it'll probably be one of my favorites until the next two come out and perhaps take its place, but I can still acknowledge it's faults. Phasma was meant to be used more, but it's like they forgot about her in the script writing sessions and that's terrible. She did nothing that a stormtrooper couldn't do for the movie. There were no scenes of hers that a stormtrooper couldn't have been used in. Hell, the fight with Finn I mentioned, the one she should have been in, was in fact fought by a stormtrooper. She's supposed to be the female stormtrooper equivalent of Boba Fett in terms of her design and her use in the film, but for the life of me I can't see why. She's important, but never ever used effectively. That's bad.
As for Poe, he was more of a plot device than he was a character. He initiated the delivery of the map piece to the resistance, he deus ex machina saves Finn and the others at Maz's castle, and he blows up Starkiller Base because someone had to do it. The movie set up why all of this stuff had to be done by Poe, certainly, but the writing for each of these plot points were poorly done if they meant to use Poe in any substantial way as a character and not just a device. It's clear that he's meant to be an important character, much like Phasma, but he suffers the same lack of use as Phasma as well. They just sorta forgot about him. He pushes the story along because it needs to be pushed along. As a character though, he needs a lot more development.
Those are my reasons as to why they needed more screen time. Without their screen time, they don't seem as important to the plot as they should be, or at least as important as the movie suggests they're meant to be.