The following things might be an explanation:
0) not updated Calibre
1) your EPUB file does not validate or it has a DRM
2) your XHTML exlibris page has some problem (have you tried using one of the supplied samples?)
3) there is a bug in exlibris that your book triggers.
If the latter is the case, if you want you can send me the EPUB file so that I can look at it and maybe fix the library.