Check the lead - Wriggle it around to see if you can get it to fault.
USB - Ensure that the contacts are clean and that it is firmly in place, the cheaper usb sticks over time don't lock into place as well as they did when first purchased.
Headunit - Try other USB sticks and make it fault with the new one. Also check the contacts or the connection (if there is an USB extension) of the USB Input.

I know I may be irrelevant but the format of the files can effect the performance of the unit, for this you will have to check with a dealer of pioneer or the user manual. (Don't expect to much from the dealer as you purchased it online.

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WGM