I have many third-party plug-ins installed on my laptop and desktop. They have different install paths; the laptop is a to-the-letter Cakewalk installation and the desktop is highly customized, but I have not experienced this problem so can't offer any suggestions on how to solve. IIRC Slate uses iLok, yes? Could there be some kind of secondary authorization, like being keyed to a specific hard drive or machine ID?
Do Kylotan's suggestion first - make sure the plug-ins show up in the plug-in manager. But consider contacting Slate support. Perhaps they have encountered this issue before, and have a simple solution.
If all else fails, maybe you could obtain the presets used for the plug-ins in particular projects, load your plug-ins to replace the identical ones in the project, then load the presets. But it really seems that shouldn't be necessary. SONAR can load projects from collaborators where you have mutually owned plug-ins. The question is what about specific plug-ins complicates that process.