In order for two compatible Pokémon to breed, they must be supplied with the neccessary item requirements. There are three different item requirements for each type by default. A Pokémon's item requirements will only depend on its primary typing (E.g. Gastly will only require the items from the ghost category). If you were to breed two different pairs of gastly's you would need to get two times the amount of items then if you were to only breed one pair.
The item requires corresponding to each type are as follows:
Once both Pokémon have been supplied with the necessary items, the breeding process will begin by selecting the green tick.