There can be several reasons for this and the following solutions have all resolved this issue at one time or another.
1. Make sure you have the latest iLok/pace driver for your OS. New drivers can be dowloaded from www.ilok.com or www.paceap.com.
2. Make sure that you have the correct version of the plug-in you are having problems with installed. ie, one that matches your version of the audio program you use like Pro Tools 7.3.
3. Pro Tools users should make sure they have the plug-in version installed that matches their Pro Tools system (HD or LE). For example “Smack!” is the HD version but “Smack! LE” is for the LE or M-Powered version.
4. Remove any iLok cards that may be inserted in the back slot of the iLok.