In the documentation for new AssetManager class's nesting functional you have an error.
You need to add the created sub-asset managers into the main manager and then enqueue the assets in those.
* <strong>Nesting</strong>
*
* <p>When you enqueue one or more AssetManagers to another one, the "loadQueue" method will
* oad the Assets of the "child" AssetManager, as well. Later, when accessing assets,
* the "parent" AssetManager will return the "child" assets as well - just like it returns,
* say, the SubTextures from a contained TextureAtlas.</p>
*
* <p>The main advantage of grouping your assets like this is something else, though: it
* allows to remove (and dispose) a complete group of assets in one step. The example
* below loads the assets from two directories. When the contents of one of them are no
* longer needed, all its assets are removed together.</p>
*
* <listing>
* var manager:AssetManager = new AssetManager();
* var appDir:File = File.applicationDirectory;
*
* var redAssets:AssetManager = new AssetManager();
* manager.enqueueSingle(appDir.resolvePath("textures/red/", "redAssets");
*
* var greenAssets:AssetManager = new AssetManager();
* manager.enqueueSingle(appDir.resolvePath("textures/green/", "greenAssets");
*
* manager.loadQueue(...); // loads both "red" and "green" assets
*
* // ... later, remove all "red" assets together
* manager.removeAssetManager("redAssets");</listing>
In the documentation for new AssetManager class's nesting functional you have an error. You need to add the created sub-asset managers into the main manager and then enqueue the assets in those.