Did you upgrade your device lately ?. Did you follow the upgrade-path steps ?.
If not .. you probably have a old IPS engine running on your machine. Download the new IPS engine and install it on your machine.
You can find this under SYSTEM->FORTIGUARD. ... and offcourse when you are using VDOMs its onder "Global -> SYSTEM-> Fortiguard"
Should look like
IPS Definitions Version 14.00612 Upgrade DatabaseIPS Engine Version 4.00035 Malicious URLs Version 2.00183