Some music players struggle playing some mp3s due to various factors including:

• Album cover images which were too large.
• Directory name lengths.
• File name lengths.
• USB formats.
• USB size.

Here is a list of recommended mp3 standards to follow.

• Max directory depth: 8
• Max directories: 500
• Limit file/directory names to 40 chars
• Max files per directory: 4,096
• Image file size limit: 512 Kb
• Image size limit: 8192 W x 7680 H pixels
• Progressive format image size limit: 1600 W x 1000 H pixels