Your field battery just doesn't have the capacity. At best, you could get one full charge out of it. Even fully charged it only has approx 7000 mAh of capacity and to charge an almost empty 3600 Electrify you're using half of it, or more as there is loss between supply and what's going into the battery. As your Electrifly batteries charge the voltage of the field battery drops to the point that the charger can't do its work. Try it off of a car battery and see what happens. More than likely your batteries will charge up fine. I've even had this problem charging large LiPo packs from a car battery, the charger complains that the input voltage is too low. Guys who are really "into" the electrics will often use a DC power supply, for the charger, plugged in to an AC outlet at the field or have a deep cycle battery they bring for charging.

P.S. It's possible your field battery is defective. Did you charge it before trying to charge the Electrifly? Even so it won't be very satisfactory as a source for your charger due to its limited capacity.

Edit: Spelling correction.