Choosing the right roller brush
A motorized roller with stiff bristles is what actually agitates carpet fibers enough to pull embedded dirt to the surface. A soft roller designed for hardwood will glide right over the top of carpet without disturbing much of anything, which is why the same vacuum can feel great in one room and disappointing in another if it's using the wrong head.
If your vacuum came with swappable heads, check which one is attached before assuming the machine itself is underpowered. A lot of frustration with carpet performance actually traces back to someone cleaning wall-to-wall carpet with the hardwood-tuned soft roller left on from the last room.
