Let us say I have an EC2 VM which we will call V in security group S1. I have an EFS mount point which we will call M in security group S2. Everything is in the same VPC, on the same subnet, in the same availability zone. I have allowed TCP port 2049 inbound on S2 from S1. Both SGs have outbound everything. Yet V times out when attempting to mount M. Cloudwatch shows REJECT OK from the IP address of V to that of M. Everything looks to me to be as the docs describe it ought to be but I am obviously missing something, where else can I look to dig into this? 
Timeout when mounting EFS on EC2
networking;nfs;timeout;aws