Graphics.ExecuteComputeBuffer only works the first time

I am trying to execute multiple command buffers but the second one always outputs a blank texture. I have tried putting everything into a function that creates and destroys it's own command buffer but it still only works the first time the function is called. The second render texture is always blank. Any clue as to why?

 GL.LoadProjectionMatrix(GL.GetGPUProjectionMatrix(textureCamera.projectionMatrix, true));
         GL.modelview = modelToExport.transform.localToWorldMatrix * textureCamera.worldToCameraMatrix;
 
         CommandBuffer cb = new CommandBuffer();
 
         // Alpha
         cb.Clear();
         cb.ClearRenderTarget(true, true, new Color(0,0,0,0));
         VertColorUnwrapMaterial.SetInt("_AlphaOnly", 1 );
         cb.DrawRenderer(modelToExport.GetComponent<Renderer>(), VertColorUnwrapMaterial);
         Graphics.SetRenderTarget(alphaRT);
         Graphics.ExecuteCommandBuffer( cb );
 
         // Color
         cb.Clear();
         cb.ClearRenderTarget(true, true, new Color(0,0,0,0));
         VertColorUnwrapMaterial.SetInt("_AlphaOnly", 0 );
         cb.DrawRenderer(modelToExport.GetComponent<Renderer>(), VertColorUnwrapMaterial);
         Graphics.SetRenderTarget(newRT);
         Graphics.ExecuteCommandBuffer( cb );
 
         cb.Release();
         cb = null;
 
         RenderTexture.active = newRT;
         Texture2D newTex2D = new Texture2D(512, 512, TextureFormat.ARGB32, false);
         newTex2D.ReadPixels(new Rect(0, 0, newRT.width, newRT.height), 0, 0);
         newTex2D.Apply();