They probably lost their pair bond when you bought them. Here is what I would try switch the male and female around give the big tank. Then feed her up real well for a month or so this will also let her claim the tank as hers. Put in the divider and secure it well rhen put the male back in. Make sure the divider has a hole in it only she can get through. With thisaproach they should spawn fairly quickly. Always have the divider there for her protection from him. Another option would be find a smaller male to put with her as they will still spawn. A lot of male Cichligs are female agressive.
Good luck hope this helps.
