Hmm, that "Analog" servo name sounds confusing... it's digital it you are hooking it up to a digital output.
That said, maybe that particular servo has an issue with input pulses that are 3.3V? Sounds like @guru_florida's works so some must be ok.
Any chance you can try the servo on an Arduino to see if the servo definitely works, and then if not on the Spark Core, you could try to level shift the 3.3V up to 5V for the signal pin of the servo: