if the pony isn't moving it's head while building imposition the martingale can't do that much for you. take a deep breath and remever that the more nervous and tense you get will reflect with what the pony underneath you will show. if you stay calm the pony will too.
My horse does that aswell, she thinks she can't just take off. I lunge her then get on and walk her around the arena and walk for about 10 minutes. I start to trot and then make her walk again. Your horse needs to know who's boss and who needs to control their speed.