Create ML not recognizing all samples

Hi folks! My sound classifier is not picking up all the sound samples in the subfolder. They are all .flac .mp3 .wav and .m4a files. I've also double checked that file type is possibly not the reason some samples were not recognized. Since Create ML is a complete blackbox, could anyone tell me how should I pre-process my sound samples so that they could all be recognized by the data loader? Thanks!

Create ML not recognizing all samples
 
 
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