When I try to associate an additional meter to a property, the meters that I have associated previously seem to disappear. What's going on?

To associate a single new meter to a property without affecting other existing meter associations on that property, use POST /association/property/(propertyId)/meter/(meterId).

To associate a group of meters to a property and replace all existing associations, use POST /association/property/(propertyId)/meter 

Note: When you use the group associate call, the list of meters you specify in your call will override any existing associated meters. In other words, POST /association/property/(propertyId)/meter performs a "replace" not an "add".