are they a pair alone in their tank? If so, separate them with a divider, most people use eggcrate. Condition them and give them good food for a while, until the female puts on some weight again. Then do a water change with colder water and put them back together. Or do a w/c with RO or some other soft water (small change, don't want to deplete the kh). There are a number of ways but you can get them to spawn by simulating a rainstorm.
Last time we had a tornado here all of my pairs spawned. They were well conditioned though beforehand