AWS EC2 SSH Login Keeps Failing - Permission Denied (Public Key)

This is really starting to bug me now. I'm working (testing) on a new project in AWS EC2, I can successfully create a LAMP stack and get FTP working as long as other processes I'm playing with, I always seem to get myself locked out of the server? I.e. when I try to SSH back in, I get permission denied (public key). This has happened about 4/5 times now.

The private key on my machine is chmod 400 and never changes. The ftp server is still working, and other services are still working, but I still can't get access.

Does anyone have any ideas?

Thanks in advance