Why is mount failing silently for me?

Normally mount doesn't return 0 if there have been problems. When I had a similar problem, the reason was that systemd unmounted the filesystem immediately after the mount.

You can try strace mount /dev/xvdx /vols/data5 to see the result of the syscall. You can also try mount /dev/xvdx /vols/data5; ls -li /vols/data5 to see whether something is mounted immediately after the mount command.