Try lowering the velocity of the emission. Usually with this kind of thing the particles are going so fast they wiz right by the destination before the forces can have any kind of effect.
Alternatively increase the intensity of the attractor.
Finally you can jack up your air resistance.
Once inside the box you will have to add a friction field that encompasses just the contents of the box. Gradually increase those forces as the particles enter and that will slow them down. Remember you can animate the intensity of the forces - so you could start out with a crazy high attractor force to make sure it grabs those first particles and then lower it down as it takes hold and brings those particles closer to their destination.
And because you are firing downwards you might want to turn off the project's gravity (every project has some unless you say otherwise) as this is adding to the particles voom.