A couple of things...
First, when you say you can see many VST plugins in your folder, are you referring to the Windows Explorer folder or to the one you get under the Effects/Audio Plug In Manager in Audition? If they're showing in Windows Explorer only, you have to got to the Audio Plug In Manager (right at the bottom of the Effects menu), tell Audition what folders to look in, then scan those folders. Any working VST/VST3 should pop up.
Second, if you're talking about older effects that might be 32 bit, note that Audition only natively uses 64 bit plug ins and you'll need an adaptor to run the older ones. Most people use one called JBridge (jBridge ( for Windows ) | J's stuff ) which can be a bit of a fiddle to set up up but works well once you get it going.