Why do only some of my sprites generate preview images?

I'm trying to load all the sub-sprites in a 2D gridded sprite and make them available o na menu. Since there is no way to convert a sprite directly to a texture2D that I could see, I thought I'd try AssetPreview.GetAssetPreview(..)

Problem is that it's half working and half not. Some of my sprites seem to get proper preview images while others seem to be blank. I have checked and all sprites DO show up when used on screen.

This is my code

 void Update () {
         if (clearPreviews){
         LoadSpritePreviews();
         clearPreviews=false;
     }
 
         if (tilesetSprite != _oldTilesetprite){
             _oldTilesetprite = tilesetSprite;
             // load tiles and tielset image
             string path = AssetDatabase.GetAssetPath(tilesetSprite);
             Object[] rawAssets = AssetDatabase.LoadAllAssetsAtPath(path);
             _tilesetImage = (Texture2D) rawAssets[0];
             _sprites = new Sprite[rawAssets.Length-1];
             System.Array.Copy(rawAssets,1,_sprites,0,_sprites.Length);
             LoadSpritePreviews ();
         }
     }
 
     private void LoadSpritePreviews(){
         // laod previews
         _spritePreviews = new Texture2D[_sprites.Length];
         for(int i=0;i<_sprites.Length;i++){
             _spritePreviews[i] = AssetPreview.GetAssetPreview(_sprites[i]);
         }
     }

I thought maybe it was a timing thing, that the asset load was happening asynchronously, so I added a check box i could check to force it to regenerate the previews, but I still get the same results :(

Here is a picture of my selection grid of the preview images:

This is what the sprite importer looks like in the project window, so apparently the editor is successfully creating previews for all the sprite images..