ProxyCommand for ssh jump failed for one machine but works for others

I have ~/.ssh/config file on my local machine as:

Host head
  User Marry
  HostName xxx.xx.xxx.xx

Host machine1
  User Marry
  HostName xxx.xxx.xxx.xx
  IdentityFile /home/Marry/ssh_head_keys/id_rsa
  ProxyCommand  ssh head nc %h %p 2> /dev/null

Host machine2
  User Marry
  HostName xxx.xxx.xxx.xx
  IdentityFile /home/Marry/ssh_head_keys/id_rsa
  ProxyCommand  ssh head nc %h %p 2> /dev/null

Host machine3
  User Marry
  HostName xxx.xxx.xxx.xx
  IdentityFile /home/Marry/ssh_head_keys/id_rsa
  ProxyCommand  ssh head nc %h %p 2> /dev/null

Host machine4
  User Marry
  HostName xxx.xxx.xxx.xx
  IdentityFile /home/Marry/ssh_head_keys/id_rsa
  ProxyCommand  ssh head nc %h %p 2> /dev/null

I can successfully do ssh machine1, ssh machine2, ssh machine3 my local machine, and go to machine 1-3, but for machine4, the command ssh machine4 failed with:

ssh_exchange_identification: Connection closed by remote host

How could this happen?

UPDATE: follow muru's advice, remove 2> /dev/null, I got the message when ssh to machine4:

bash: nc: command not found
ssh_exchange_identification: Connection closed by remote host