If you're referring to the first Hercules: Prince of Power series, there are a couple of things to note about that. Firstly, that series is set in an alternate future, centuries from now, and is therefore non-canon. Secondly, Frankie was never shown to be capable of reverting to human form on her own. Rather, she just turned up in human form on the last page of the story, stating that Galactus had given her some time off.