How to reproduce:  The spell checker`s AutoSpellCheckControl property must not be set. Add a ContextMenuStrip with several items and use it with a standard text box. Then perform a spell check with the RadSpellChecker control, e.g. like this: 

Me.RadSpellChecker1.Check(Me.TextBox1)

Then right-click in the text box, the context menu will not be shown.

Workaround: use RadContextMenu
https://docs.telerik.com/devtools/winforms/menus/contextmenu/context-menus