Sometimes a generation does not finish successfully. When a generation fails before delivering a usable result, the credits reserved for it are restored to your balance automatically.
A prompt can also be blocked by a model's safety system — for example, when it includes real brand names or real people's names. Try making your description more generic and generate again.
If a failure is not corrected automatically, contact support and we can review it. Not every unsuccessful attempt qualifies for a credit restoration; support can help review your case.