The romaji-to-kana coverter on Renshuu doesn't seem to support all of the shortcuts for kana + small vowel combinations that can be typed in an IME or Jisho, which save keystrokes and are a bit more intuitive (IMO) than using x/l to force individual small kana.
These combinations (and their ゛ counterparts) don't parse: クァ/クィ/クェ/クォ (kwa/kwi/kwe/kwo) スァ/スィ/スェ/スォ (swa/swi/swe/swo) ツァ/ツィ/ツェ/ツォ (tsa/tsi/tse/tso) トァ/トィ/トゥ/トェ (twa/twi/twu/twe)
edit: テャ/ティ/テョ (tha/thi/tho) work and デュ (dhu) works but テュ (thu) does not. I'm not sure that combo is even in any words, but it's odd that it's left out of the set.
I just today used swe in a listening vocab quiz where sweden came up. スウェーデン is typed by entering su we - de nn. It took me 3 tries to figure out how to input it after su ue - de nn didn't work. Maybe you could try this kind of pattern for the mentioned non-parsing kana and see if it takes, gillianfaith?
Edit: just realized that my usage was スウェ, which isn't the same thing as what you are pointing out :/.
