Sandisk iXapnd 128GB - trying to play music on iphone , missing files

Hello there everyone, I just recently purchased a Sandisk iXpand 128GB device. My primary use is to play extra music on my iPhone. I transfered about 3500 songs onto the USB, all .MP3 files. When I plug in the USB to my iPhone and use the iXpand app to view the files, it only shows me about 400 of the 3000 MP3 files. Why is it not showing me all of the music files on the USB? I cant figure this out. They are all the exact same file type (.MP3) so why wouldnt they all show up? 

I would appreciate any help I can get here. Thank you so much. 

Same here, any help much appreciated.