It's weird because when Risa joined Tsunku said she shined during the recording, but then she wasn't really praised for her vocal ability afterwards. To be fair she joined when there were a lot of Senpai in the group and by the time they started to graduate there was Mikitty Aichan and Reina ready to be the power vocalists. Although I wonder if Tsunku doesn't prefer the deeper voiced girls as front singers and prefers them as 2nd tier to balance the main singers or something?
Then there is the fact that she wasn't as popular. I watched the audition and I know Aichan shone the most with maybe Ogawa and KonKon next (Ogawa was in the audition the obvious best and KonKon the worst but Tsunku seemed to see something in her and she was added anyway) Aichan won for the most improved being that I remember her being lectured and crying but then practicing harder than everyone and coming back stronger for it. That's why I fell in love with her
Risa was like a random addition it seemed. She also had this baby face and looked young for the longest time. Even the rookies who were not much older when they joined looked more mature when they were added. Risa eventually blossomed into a beautiful woman but that wasn't until Yossi became leader. I think that's why she wasn't popular. From the day she was added she seemed to be lost in the sea of girls, falling easily to the back.