The flower needs to be fully mature to have pollen and an ovary capable of receiving it. That white flower isn't even open all the way yet. See how it's still green in the middle and the petals are folded in.
I'd wait until it looks almost "blown" then carefully cut the flower in half and see if it has pollen (stamens) and an ovary (pistils?). Then wait for the next flower, let it get fully open and mature, then try crossing them again.
