The same thing happened with mine, but it might have been because they were too young.. they were both 3 years..
Also
I used the love potion too.. and only one of them had hearts above it's head after i used it.. i used it on the other one.. but she didn't have the hearts..
This confuzzles me..
So I guess I'll just have to wait and see what happens..
Reply if you know anything about breeding two V4's