Hi,
Check in transaction against your billing type Posting block is flagged or not. If posting block is flagged then accounting document will not be created automatically. To release you have go to transaction vf02 and release the flag. If not flagged then it will create automatically. Though it is not marked for posting block if any G/L is not assigned in determination or If the finance period is not open then also it will not post automatically.
In your case due to some issue like finance period is not open so accounting document would have not created. But after that the finance period is open then when you go in to the same document this time accounting document would have got created.
Regards,
Senthil Venugopal