In the AP workflow page, in detail section, wheh I enter project information adn click save. it is e

In the AP workflow page, in detail section, wheh I enter project information adn click save. it is erroring out saying code_combination_id cannot be null (ie.e) it is not deriving account. But if I enter some account and also enter project information, it is saving but not deriving account even though we customized AP project account generator. One more thing... If I click approve without account number, its is deriving account using project account generator and saving it. Please reply why save is erroring out and not deriving account using project account generator.