Good question. I think the veins and capillaries downstream of them might just be too small. Doc said eventually they'll dissolve on their own. I started the 81 mg aspirin regimen just in case a little thinner blood would help dissolve. It's been almost 2 years. The bigger clot seems to be gone but a newer small one appeared in my calf. And the other smaller clot is still there. Most can be felt since they are right below my skin. Yes, genetics are a factor. My mother's side was riddled with clots.