I'm sure if this is a popular opinion but I love the way magic works. For me, I believe magic manifests in the way that is most effective for the spellcaster. For instance, when Paige cast that good luck it caused her to orb into the leprechaun's realm but since Piper & Phoebe can't orb how would it have worked for one of them. The spell to track the Banshee turn Prue into a dog but I believe it was because three people cast the spell since it would make no sense for the spell to work that way for a lone witch because they wouldn't be able to fight the Banshee in that form. Or maybe the spell would've reversed itself once a lone witch found the demon. Even the Give Me A Sign spell worked on Prue, though intended for Piper, because the spell was worded to help her sister free her heart and Piper already knew what her heart wanted even though she wasn't ready to admit it.