Shabbos 24 – שבת כד

The Gemara questions whether Chanukah or Rosh Chodesh are mentioned in Birchas Hamzon, but both days are certainly mentioned during Tefillah.  However, there seems to be a discrepancy between them; if one omits Rosh Chodesh (Yaaleh veyavo) during Tefillah he need to repeat, but if he omits Chanukah (Al hanissim) he doesn’t – why?