Lian,
We all know what it's like to run into an aggravating problem like this when you're learning a tool like LabVIEW.
The decoration you added is considered to be "on top of" the buttons by LabVIEW, probably because you dropped it onto the panel after you created the buttons. You can set it to be below the buttons by selecting it (arrow tool), then using the Reorder button in the toolbar (the little loop-of-arrows button farthest to the right) and selecting "Move to Back".
Best Regards,
John Lum