User blog comment:Hidden Scorpius XI/An ever-present detail/@comment-3146930-20150120213220/@comment-3257939-20150121063350

It's very simple like Darkcloud is saying. Pyrrha's Semblance is the ability to control magnetism and in order to do that: She needs to create a conduit.

Since it's polarity then it depends on electricity to work so it needs a conduit to travel from place to place. We saw when Pyrrha controlled all those cans that she sent a wave of electricity through the soda (liquid = conduit) in order to reach the cans with her polarity and then she was capable of controlling it so easily. We also saw that she touched Mercury's weapon before she was capable of controlling it because metals are also conduits.

However, we saw that she is still capable of controlling things without touching it but as we saw when she lifts Jaune's shield: She needs to concentrate. Since there is no conduit, she needs to create one herself by concentrating her aura and reaching the object through her own Semblance in order to move it but this is harder to do so. The conduit also does not last forever so Pyrrha can't just touch the object to control it whenever she wants in the future.