Форуми / Cotonti / General / Markitup partially not working (Siena 0.9.6)

Then start editing a page and open JavaScript error console in your browser (Shift + Ctrl + I in Chrome) and see if there are any errors. Probably the editor doesn't display because JavaScript gets broken on page.

And another thing. If that page already has "none" selected as a parser (yes, you can also select a parser for a each particular page) then it won't display any editor until you select a proper parser.