Unload Assetbundle

I am facing problem with unloading assetbundle. I have created couple of assetbundles (each for level). When i want to start second level, i unload first assetbundle by using unload(true). also i have tried Caching.cleanCache() but still its not deallocating memory allocated for first assetbundle. I am not getting where i am going wrong.

Here are the things i have done :

   1. Loaded asset bundle with www.load(url).
   2. loaded prefabs in assetbundle and assigned it to script to use it runtime.
   when i start loading 2nd level
   3. remove attached prefabs in script.  assetbundle.unload(true); Caching.cleanCache(); Load assetbundle for second level
   4. repeat the same procedure.

Still its not deallocating memory.