After you kill the mosquito, as you go to leave the palace, you will have an encounter. At that time, someone will mention it would be a good time to check the counters again. That's your cue for Cindy.
If she's not at the Rillia Counter at that point in the game; then odds are you either skipped donating, donated the smaller amount, or perhaps could not afford to donate at that point in the game. (You may have selected the full donation back then, but were unable to afford it.)
The common understanding for Cindy is that you donate the full amount the first time you visit Rillia Counter, and when you return there not long after the Mosquito event, she will be waiting. I'm not aware of any other requirements beyond those, so I can only assume you missed the donation in one way or another.