avassetwriterinput readyformoredata is NO when audio sample buffer size is changing

Question

I got readyformoredata always no if audio sample buffer data size is changing. But if set sample buffer size to changeless value, readyformoredata becomes yes again.

Does anybody know why?

and here is my audio recoding settings:

    NSDictionary *recordSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                [NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
                                [NSNumber numberWithFloat:44100.0], AVSampleRateKey,
                                [NSNumber numberWithInt: 2], AVNumberOfChannelsKey,
                                nil];

Another possible clue is if set AVNumberOfChannelsKey to 1, readyformoredata could be yes, but appendbuffer return failed.

any suggestions will be appreciated~ thank you~