Q: If the soul is part and parcel of Krsna (BG 15.7) and given Krsna is in the spiritual world and everyone in the spiritual world is infallible (BG 15.16), how is it that the soul has fallen into the material world in the first place?
                                                                  As Krishna's extended parts and parcels, the jiva has qualities like Krishna. One of those qualities is free will. Krishna has unlimited free will, whereas the jiva has tiny free will.
                                                                  When utilized properly, the jiva's free will is lovingly directed towards Krishna's service and seeking His happiness.
                                                                  When free will is improperly exercised, it leads to asat-trsna, or hankering for enjoyment separate from Him.
                                                                  Spirit soul falls into the category of marginal energy of the Lord, namely jivas, due to their tiny nature, may come under the influence of inferior material potency and under the influence of spiritual potency. They are free to choose between loving Him or pursuing selfish illusions.
                                                                  Jivas constitutionally belong to the 'marginal' category eternally. When the jiva soul chooses to misuse its minute independence, it falls down to the material world.
                                                                  You can also refer to Digest 72 for more detail.