Well.
I tried again with the same couple. Ghost potions again. This time, they got their ghost baby.
Only thing that springs to mind is the "when" of conception. The first time, they got the chimes on the second Try for Baby. But they were both still under the effects of the ghost potion for another hour after woohoo ended. The second time, they got the chimes on the first try.
Either that, or the effect of a ghost baby with the potion is still at 50% for both parents? It was odd.