Auto_Blend Layers not working

Using Ps CC 2014 and Win7:

I have an image with Background = transparent and two layers, each containing an RGB/8 image.

Following instructions as per the Help file, and with both layers selected, I click Edit/Auto_Blend Layers and selecting Stack Images and Seamless Tones and Colors results in the image in the topmost layer disappearing!

I have tried this using different images (including the exercise in Adobe Ps CC Classroom in a Book on p150) with the same result.

Could anyone help, please?

Thanks

Paul