I think Achilles give problems for several different reasons, would hard to get corrective advice on forums. Achilles have low blood supply, so can take a while to repair. Good news is, they almost always do repair.
i used to run off-road fir several years, not big mileage, just 3 x 5 miles each week for 7 years. by year two, I had big lumps in birth my Achilles, very tender and very stiff after being stationary. After seeing a good spirits physiotherapist, I turned out it was my fault along. I needed to stretch "properly" after runs. I also needed to use a roller to massage the knots out of my calf muscles. I also slightly over proneated, meaning ankles, Achilles, knees, hips etc were not lined up correctly. A pair of scholl orthotic insoles sorted this out. Within six months, my Achilles were 100% good again.
So for me, three things sorted it out. Pre and post stretching, regular self calf massaging and scoll orthotic insoles.
hope you get it sorted.