Try these options:
1. Flush the collector (approach > interact > select flush) > milk Kelpie x Bunny then fill the collector with it (1-2 litters or more, to be able to get a new milk cup) > pick it up and try again thru the town or town's entrance.
2. If nothing works, try sleeping (or do something else in the mean time, like level up chars) until next week, then try again. Crowds are reseted/respawned once a week. Or on game restart