I read about all these other people not their fish taken and now... it's happened to me. She only took 10 fish and I didn't get the achievement for handing in 20. The remaining 10 fish are still in my inventory. Tried refreshing but she just wants me to do the next step of the story line. Is there anything I can do about this?

Unfortunately not, the second achievement for handing in Splittifish was only available for a limited amount of time.

Poo. And I really wanted that title. Oh well, Thanks for your help :)