[NTG-context] combining character (unicode 0364)

Dear list, a question concerning combining characters: I try to make a letter out of a small elevated „e“ (unicode 0364, combining latin small letter e) with „a“, „o“ and „u“ using Junicode with Wolfgang Schuster’s typescript. In Fontforge the mark positioning lookup looks good to me, but ConTeXt somehow seems to ignore the anchors in the font. The „mark“-feature is enabled in the typescript. Maybe someone can give me a hint how to get this working? Thanks in advance! Daniel