When I click on it it says spell check complete but it has not checked the text
Does the text you are checking actually have a language associated with it?
To activate the spelling checker press F7 or select Tools>Spelling. This works with Windows 7/LO 6.0 and I hope the procedure for Linux etc. is similar.
If any of your text is selected (even a single letter), only the selected text will be checked. If the message “The spell check is complete” appears when the whole of a text has clearly not been checked I usually find it is because something has accidentally been selected before starting the check (a little slip with the mouse is all it takes). To avoid this I would recommend the habit of selecting the whole of the text you want checked before pressing F7.
I would recommend taking a look at pages 52 et seq. of the LibreOffice Writer 6.0 Guide which you can download as a PDF, free of charge, from the LO documentation page English documentation | LibreOffice Documentation - Your documentation for LibreOffice.