Swinging is like the dessert course in a multi course meal. It evolves within a relationship... which are the main courses. Swinging can be a part of a relationship, but is never the biggest or most important part of one.
It is very common for the main courses to become so filling, there is no room left for dessert, and even though a couple swings (or a single even) it is not high enough on their priority list for them to be as active as they once may have been! Intuition897 comes to mind here, at least as it pertains to this forum!.
Also, I think it is impossible to screen for a mate open to swinging because that person may have never been inclined to enter the lifestyle with anyone else in the world but you.