Probably a lot of things inside the character blueprint.
Aside from that, which you just have to learn about and it is a lengthy process, are you playing this animation as a montage?
If so, it is very likely that the montage is slotted to only play on the upper body by design.
What I would do, is to create an animation curve for the hip rotation, and create a system that would apply that rotation once the montage is played based on the montage curve value so that it is automatically perfectly synced.
You will still sacrifice some of your twist at the legs, but at least your character can move about while attacking.
Given those are pretty advanced techniques, I would suggest you start with watching the unreal live training videos for animating the Paragon characters. They are a great overall deep dive into the whole process and what it entails.