Separate streams
Many platforms store high-resolution video and audio as separate streams. The processing service must fetch both and mux them into one playable file, which is why 1080p and 4K take longer than 720p.
Choose an available quality
1080p, 1440p and 4K appear only when the source provides them. If a quality is missing, the original upload simply does not contain it.
Playback checks
If a file has no sound, try a compatible MP4 option or an audio format — some high-resolution variants are video-only. If playback stutters, 720p or 1080p may suit your device better.