Peter, one thing you may try is mobility work on your calves - just below the knee, as well as around your achilles (ie - at the ends of the muscle). Rolling these areas out with an agressive foam roller or even using a barbell can really help break up some tightness and improve mobility over time.
I have also used a kettle bell (or simply holding any weight will work) to sit in the bottom of a squat, centering the weight over one ankle at a time to slowly stretch the achilles. I broke my left ankle two years ago and there is a notable difference in my flexibility from left to right. These methods have helped me close that gap significantly and I don't see any reason why it can't work bilaterally as well! Good luck!