Apart from V5.
The V5 can only mate with another V5.
But I would assume that V3 and V4 could mate.
The love potion will only work onnce they are adults, and are old enough to mate, though.
Have you been treating it right?
The bonding percentage will go down if you miss the 'training' parts.
Maybe that was the case?
Or you havent been looking after it properly.