How do you cross breed swordtails with platies?

ok, so im frustrated..i read EVERYWHERE that swordtails and platies (maculatus) "interbreed freely" lolol...i have 20 gallon tanks..heated 78-80f. nothing else in them. so when i go to cross breed i make sure theres no available mates of the same species so that way there forced to mate together...i tried male sword to female platy..(i usually do in one 20 g tank 1 male swordtail with 3-6 female platys so he has a choice) NOTHING!!! they dont even acknowledge one another just swimming up n down the glass doing whatever, no courtship displays how swords usually do by trying to "stab the female". NOTHING!!!!! so in my other 20 gallon tank..same heated to 78-80 F. no fish in it..i use 1 male platy to 3-4 female swords..NOTHING again!!!! what do i have to do to get them to mate?? there both of the genus xiphophorus. all the books and internet articles say they "interbreed freely".. well has anyone had luck in hybridizing them??? i want to make hybrids, i seen aweosme pics how the male f1 hybrids of the cross come out with tiny curved up swords and neat patterns not seen of. i wanna experience them. so what set up would you recomend for me......male platy to 3-4 female swords...or male sword to 3-6 female platys to acheive them to mate??..like i said every time i go to mate them i make sure there the only fish in the tank with no mates available of the same specie so theyd be forced to mate..is this normal why they dont even acknowledge each other ever??? please help me out with your experiences so i can get them to mate and produce hybrids!! thanks!!!!!