Build assetbundle successfully but cannot instantiate it

My unity is 5.2.0.I have built assetbundle using a simple cube and the size of the file is 45KB. And when I download it with WWW class, the www.error message shows nothing. But when I instantiate the cube, unity tells me the result is null. I assume there's something wrong in the building progress but I cannot find the reason.

SelectAsset is all the assets I selected in Project panel. I get their path one by one and put it into resourcesAssets.

 for (int i = 0; i < SelectedAsset.Length; i++)
 {
     sourcePath = AssetDatabase.GetAssetPath(SelectedAsset[i]);
     resoucesAssets[i] = sourcePath;
 }

After that, I use BuildAssetBundles to build assetbundle. Do I have to use AssetBundleBuild in this condition?

 BuildPipeline.BuildAssetBundles(targetPath);

Here is my downloading code. I download manifest file first, and www.error shows nothing. It's correct.

 WWW www = WWW.LoadFromCacheOrDownload(mainUrl, 0);
 yield return www;
 
 AssetBundleManifest manifest = new AssetBundleManifest();
 
 if (!string.IsNullOrEmpty(www.error))
 {
     print(www.error);
 }
 else
 {
     AssetBundle manifestBundle = www.assetBundle;
     manifest = manifestBundle.LoadAsset("AssetBundleManifest") as AssetBundleManifest;
     manifestBundle.Unload(false);
 }

Then I download the assetbundle file with cube. In this case, wwwGoal.error shows nothing as well, but "Instantiate(goal);" tells me the result is null.

 WWW wwwGoal = WWW.LoadFromCacheOrDownload(path + realName + extension, manifest.GetAssetBundleHash(realName + extension), 0);
 yield return wwwGoal;
 if (!string.IsNullOrEmpty(wwwGoal.error))
 {
     print(wwwGoal.error);
 }
 else
 {
     AssetBundle ab = wwwGoal.assetBundle;
     GameObject goal = ab.LoadAsset(realName + extension) as GameObject;
     if (goal != null)
     {
         Instantiate(goal);
         ab.Unload(false);
     }
 }

The size of the assetbundle file is 45KB. It's right in normal cases. I have confused about this for several days. Could anyone tell me the reason?

Thanks!