Why do you want to heal them if they are not broken?
If they have good routes and return routes it seems an odd thing to want to try to change them.
One thing that may be happening is that they are not that good but you will only tell how not good they are is with a zniffer.
It is hard to say why the heal is not working as you have not said a lot about your network.
There are a few things that may be happening but one thing about heal is it uses a lower power than normal transmission -6db.
What may have happened is that the route to your edge node was established by explorer frames but the route does not work at -6db so heal fails.
Heal is an odd animal and can fail for many reasons. It generates a vast amount of traffic in large or dense networks where many nodes can see each other at -6db. It can kill itself due to the pure volume of traffic generated and any timeout at the application end.
I have a few marginal nodes at the edge of my network and the worst thing I can do is succeed to heal them. They pick up a worse route than they had before heal. Until the route fails badly and an explorer frame gets them back to a good route they are laggy and generally horrible.
I never heal them. That is my strategy for nodes that are working fine.