The area under the heat lamp is probably too hot and too bright, so they are staying away from it. With that many in a brooder that size, there really isn't anyplace else to go or anything else to do but just park it in a cooler corner and stay there. Ducklings heat requirements are different than chicks - they don't need as much heat. If it was me, I think I'd reconfigure the brooder to have the light at one side and up higher - or better yet provide them with a bigger brooder where they can get away from the heat.