But Tamao is utter fail at even *attempting* to express her feelings to Nagisa. Nagisa never once saw her as anything other than her roommate because Tamao never presented herself as anything else. Nagisa isn't a mind reader (she barely has one of her own), she couldn't magically infer that Tamao was crushing on her.
And Tamao never had much beyond "she's my roommate, therefore I must love her" as a motivation, anyhow. Nagisa might have started as just another fling for Shizuma but she grew to be the one to reopen her heart and let her move past Kaori.