Drizzles are very slow to fill, and it depends on the funnel, too. Makeshift or leather funnels will take a loooong time, but normal funnels are ok. Metal funnels are the best.
I’d say just keep playing normally while it rains, and after a little while it should have filled up a bit.
Additionally, is the 60 L tank damaged? If it’s damaged beyond purple, it’ll leak whatever’s inside of it, which will prevent the rainwater from ever pooling in the tank.