If you are not finding the JFK-AGP flights on your searches, try using June dates. These flights run from about June 1 (+/- a few days) to mid-October. I flew JFK-AGP on May 30 this year, returning June 8. Because it was considered a new route, there was a double miles (RDM, not MQM) promotion, which was some compensation for the fact that Delta doesn't use its newest and nicest planes for the route.

It's a great way to get directly to southern Spain.