i absolutely agree with the other posters. it all takes time. your intentions are in the right place, but you can't force emotions on yourself just because you feel you "should" feel them.
i do have a "super nanny"-esque idea though - have you tried spending time with only the adopted children? maybe a day out just the three of you (i can only imagine how hard that might be to schedule!) would help you get to know each other and trust each other a little better. the more time you spend with them, i'm sure, the closer you'll feel.
don't beat yourself up over this. i myself only have one child. but no one told me that carrying this child for nine months doesn't guarantee it's love at first sight - it took me several weeks to really feel that bond. that's the god's truth. i was a new mom, i was nervous and stressed out, then we had to take a trip halfway across the country right after he was born. there was NO magic moment for me! i always felt bad about that. but now i love him more than the world. besides, despite what everyone claims, i've had parents of multiples admit to me that they do have favorites. i think it's inevitable. just allow your heart to be open and let what happens happen. i'm sure the love will come and eventually you'll wonder how you ever got by without them!