I can not add the Strikethrough button to the Writer toolbar

I started by clicking Tools → Customize → Toolbars, then select Text Box Formatting option and add Strikethrough.
It appears checked along with bold, italic and others.
After that I went to the toolbar, right click → Visible buttons and Strikethrough is not available.
I’m using LibreOffice in a Windows 7 Home Premium 64 bit laptop.

Could you please help me out?

You are likely customizing the wrong toolbar. Right click the toolbar, which you want to change, and use the item Customize from this context menu. Then this toolbar is preselected in the Customize dialog and you change the correct one.