There's storm serpents and night serpents, storm serpents don't go away when the rain finishes. Also, try to find a spot with multiple Serpents and the spawner will be close by for you to activate when it turns night. Hunt on the borders of a swamp biome since the leeches will help you take out the Serpents faster and they usually have shallow water. You should be able to grab 3 Serpents a night if you find a swamp with multiple nearby spawners.