iOS cannot play "slow" or "veryslow" h.264 videos

I have encoded a video that I want to transfer to my iPad. Using FFmpeg, the settings are: ffmpeg -i input.mkv -c:v libx264 -preset veryslow -crf 32 output.mp4" When I have tried the presets veryslow and slow, I get this message when trying to sync the video via iTunes to the device: > "[Your File] was not copied to the [Device Name] because it cannot be > played on this iPad." However, I have tried encoding the same video using the preset medium and sync and playback in iOS's *Videos* app works. Is there a work-around to get the "slow" preset videos onto the iPad?