iPhone transcription not working for longer recordings

I successfully used the new transcription feature on my iPhone (ios 18.1.1), converting a 12+ minute recording of a phone call to a transcript. It did take what seemed like a number of minutes before the transcribing began, following initiation. So far, half-hour files do not appear to initiate the transcription process, even after 15 minutes. Is that an issue of file size or am I not waiting long enough?

[Re-Titled by Moderator]