How to debug when Kubernetes nodes are in ‘Not Ready’ state

First, describe nodes and see if it reports anything:

$ kubectl describe nodes

Look for conditions, capacity and allocatable:

Conditions:
  Type              Status
  ----              ------
  OutOfDisk         False
  MemoryPressure    False
  DiskPressure      False
  Ready             True
Capacity:
 cpu:       2
 memory:    2052588Ki
 pods:      110
Allocatable:
 cpu:       2
 memory:    1950188Ki
 pods:      110

If everything is alright here, SSH into the node and observe kubelet logs to see if it reports anything. Like certificate erros, authentication errors etc.

If kubelet is running as a systemd service, you can use

$ journalctl -u kubelet