Why can't I put two Pokémon in the Day-Care at once in FireRed/LeafGreen?

Question

I really want to breed!! I have LeafGreen.
P.S. I've bred in white ruby black and white 2 so I know how to.

Answer 1

Pokémon can breed in the Pokémon Day Care in Four Island. The daycare on Route 5 remains, but it is still limited to only caring for one Pokémon at a time.

You're probably using the one on Route 5. You have to use the one on Four Island

Source