Do they happen to be Mametchi and Chamametchi? Because they made it so that brother and sister can't mate. Once they get four happy faces, I think you need to connect them a couple more times until they actually mate, but that might be a different version.
They must be four hearts, not four happy faces. Also, Tamagotchis usually can only marry one or two days after evolving into adults. Violetchilluvr3 is right about not being able to wed Mametchi and Chamametchi because they are siblings. Furthermore, Kunoitchi and Gozarutchi cannot marry eachother.
I never knew that Chamametchi and Mametchi could mate each other.. I connected my Chamametchi and Mametchi together once for 200+ times and nothing happened..