Screen doesn't always lock correctly after inactivity

When my laptop shuts off the screen due to inactivity, when I come back, the screen isn't locked. I can start back to work and interact with my apps, then the screen locks after I've started back to work.

Expected behavior: The screen locks after the inactivity timeout has expired (5 minutes in my case).

Actual behavior: When I "wake up" the screen, it's not locked, but then locks a few seconds after I've started to work.

I use my laptop for system administration, including access to a production environmnent, and can't have it left unlocked.

The behavior doesn't seem to be consistent, but I'll keep testing to see if there are specific scenarios under which this occurs.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: gnome-screensaver 3.6.1-0ubuntu13
ProcVersionSignature: Ubuntu 3.13.0-32.57-generic 3.13.11.4
Uname: Linux 3.13.0-32-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.14.1-0ubuntu3.3
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Aug 8 10:55:22 2014
GnomeSessionIdleInhibited: No
GnomeSessionInhibitors: None
GsettingsGnomeSession:
 org.gnome.desktop.session session-name 'ubuntu'
 org.gnome.desktop.session idle-delay uint32 300
InstallationDate: Installed on 2014-07-28 (10 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
SourcePackage: gnome-screensaver
UpgradeStatus: No upgrade log present (probably fresh install)