[Alabala]

I am creating an ebook written in Cyrillic using InDesign. I need to embed a Cyrillic font (my choice goes to GentiumPlus) in its epub version in order to make sure it displays correctly. My problem is, Digital Editions keeps showing ??? symbols when I test the document.
For the past few days I've gone through various threads, posts and tutorials on embedding fonts and have tried different solutions. I have tried embedding my font both using the InDesign option and manually, following the instructions I found (adding the font; editing the .opf and the .css files). So far, the epub displays correctly, with the embedded font, in Calibre and various browsers (IE, Chrome) but not in ADE.
What am I doing wrong? Any observation are greatly appreciated.

Here's a part of the css file (I've also specified the font family in every individual paragraph style):

@font-face {
font-family: "Gentium Plus", serif;
font-style: normal;
font-weight: normal;
src:url ("../font/GentiumPlus-R.ttf");
}
@font-face {
font-family: "Gentium Plus", serif;
font-style: italic;
font-weight: normal;
src:url ("../font/GentiumPlus-I.ttf");
}
body, div, dl, dt, dd, ul, ol, li, h1, h2, h3, h4, h5, h6, p, pre, code, blockquote {
font-family: "Gentium Plus", serif;
margin:0;
padding:0;
border-width:0;
}

And the one from the opf file:

</metadata>
<manifest>
<item id="GentiumPlus" href="fontGentiumPlus-R.ttf" media-type="application/truetype "/>
<item id="GentiumPlus" href="fontGentimPlus-I.ttf" media-type="application/truetype "/>