ssh permissions not working, even after set to 600

ssh permissions not working, even after set to 600

I have three files in my ~/.ssh/ folder:
id_rsa, id_rsa.pub, and rootkey.pem
rootkey.pem is the .pem from an Amazon EC2 server.
I've been trying
ssh -i ~/.ssh/rootkey.pem ubuntu@blahblahAmazonEc2.com
all day, but I keep getting
It is required that your private key files are NOT accessible by others.
This private key will be ignored. bad permissions: ignore key:
/home/Andy/.ssh/rootkey.pem Permission denied (publickey).
The obvious solution is to set
chmod 600 ~/.ssh/rootkey.pem
I've done this a million times. It makes absolutely no difference. I've
tried 400 as well, no good.
I have no idea why it's not working. Anyone have any clue? Thanks!