Found this on a forum "With new 3.12 firmware, DJI had modified prerequisites for AUTHORIZE phantom arm motors - fly. If not enough sattelites in GPS or not enough warm time or not 20 green led sequence reached, your motors won't start. In addition, after get GREEN or GREEN.GREEN or YELLOW or YELLOW.GREEN sequence you must throtle 50% or higher for "semi automatic" take off."
Maybe a faulty GPS? i only have a naza lite on my Disco so no firmware updates