In certain situations, Looker won’t bring measures that are defined in one of your views into the field picker when you are exploring because we can’t be certain that those numbers are accurate. This happens because you’re using symmetric aggregates, but you haven’t defined a primary key in one of your joined view files. Usually, this comes up when you are joining two view files with a relationship that is not one_to_one, or you are using a sum_distinct or average_distinct measure. When either of these two conditions are met, you must define a primary key in the joined views.
Check each of the view files to make sure there is a primary key defined. If you find one that is missing a primary key, you can create a new one:
dimension: id {
type: number
primary_key: yes
}