Debugging Swarm status issues?

I have seen that sometimes my swarm nodes get stuck in status Pending.

Is there any way to figure out why this is?
I know that if I restart a node and it gets a new IP, Docker might fail to start du to Cert issues.
That can easily be solved by regenerating the certs… but some times that is not enough and I still get Pending nodes even if Docker is running fine on that machine.

So what do I do?