Some spells require an additional item in order to be performed, like a candle or poppet. It should also be noted that not all witches can cast all spells. The success of a spell seems to depend on the power level of the witch (or witches) casting it.
Some spells require more than one witch to make them work. This form of collective spell-casting is a principal feature of magical collectives such as the Charmed Ones.