Considering the range of different currents delivered by proven compatible chargers mentioned in the community list, and 700mA (0.7A) among them, these parameters sound fine and you were right to assume the charger would work.
Since the current sounds fine and USB charging normally should deliver 5V voltage, meaning the FP2 would probably expect that voltage, perhaps the voltage delivered by the charger is too low (4.5V?) or not stable enough over time for charging the FP2 … but you would need a way to measure that to be sure.