Make sure you have:
set the profile of the housekeeper to enabled.
set the availability of the housekeeper in their profile.
allocated the housekeeper to the property.
set the correct average housekeeping times in your unit types.
The system will try to auto-allocate the housekeeping tasks to your housekeepers based on the above, as well as check-in and check-out times, and travel time between properties to try and achieve the highest optimisation possible. You can override the allocation set by the system in Tasks-> Cleaning-> Upcoming Housekeeping Schedule