I just checked again and i cant find anything wrong with the trigger.
The only requirements for the quest to happen is that Sarah is in the lobby, and that her arrival was at least a day ago.
After that you should be able to talk to Sonya to get the pool, but only when she is working in the bank.
As a workaround you can try a previous version of the game that lets you skip the encounter in the lobby
here . You should be able to go directly to the bank when Sonya is there and get the pool. After that you can switch back to the current version as the quest doesn't have any other implications.