My soleus muscles are my achilles heel. I've had lots of trouble in the past.
When I saw a physio a couple of years ago because I couldn't run on them - one worse than the other but both painful to run on, he prescribed stretching and strengthening them 3 times a day and a gradual return to running over a couple of weeks. Had to take one week off completely. It worked, but they still play up occassionally
When stretching the soleus you need to have your knee slightly bent (unlike for the gastro), you'll feel the difference. Strengthening was to rise up onto balls of both feet, lift one just off the floor, slowly lower the supporting heel down (on a step so it goes low if you can), then rise up on both balls again.
Hope you sort it. I reckon I have problems because I have short calves compared to the rest of me - just a theory though