I noticed, that sometimes, depending on whether you favor Natsuki or Yuri more, by the 3rd poem, the other may completely ignore you based on poem order. I've had this happen once for both girls.
On my first play through, I focused on Yuri, I would show her my poems first and then the other girls. By the 3rd poem exchange, Natsuki was so pissed off with [me/my character] that she refused to even talk to me. This also happened on my last play through when I focused more on Natsuki and had Yuri very upset with the character as well.
With the other play throughs, if I saved the poem for the girl I was focused on last, then I wouldn't see any problems during exchanges. Just a pretty interesting if/else code segment setup in the design (I found it fascinating as I'm a programmer myself).