Opacity masks to hide parts of a shape behind another shape

[Natasha263500877bhe]

I hope someone can help me as I'm sure the answer is simple and yet I am spending hours now trying to do this. I am trying to follow the tutorial below, using an opacity mask to hide a part of an object behind another. Despite following the instructions there never seems to be any change when I add the mask. I have the same image on my transparency window before creating the mask, yet nothing changes. Is there something missing from these steps potentially (as I am a beginner and maybe it's assumed) that may help? Any advice would be greatly appreciated. 

Cheers Natasha 

[Monika Gause]

There is always the possibility of the GPU preview not working correctly, so please try switching between GPU and CPU view in the View menu or with Cmd+E

Then: There are options for the mask in the transparency panel: invert mask and clip. Please check that you set them up correctly. And then: can you please show a screenshot of what you did? So we can check out where you possible went wrong? Please show the transparency panel as well.

[Natasha263500877bhe]

Thank you for your speedy response. 

I will attach a screen shots of what is happening for me. 

So confused! 

[Monika Gause]

Can you try and uncheck that "Clip" option in the transparency panel?