Why won't my cd player with usb play all the songs on a couple of my usb sticks.?

if it only ever plays up 512 songs no matter storage device is plugged into it, that kind of sounds like a software limitation built into the playback machine's programming. 

i'd guess it's probably set up to create a list of every track on the storage device for itself, and then operate on that list to determine what order things get played, whether tracks get repeated and so on.  and they only allotted a finite amount of space for the list. 

because if they let the list grow to be literally *any* size, then it would be possible for some fool to plug some godawful huge number of songs in there, like, oh, 1500 say, and then the device might run out of physical memory trying to build its list and then the machine would have to show you an error message or something and it would be just horrible

so instead of showing you an error message, it just stops building the list at 512 and acts like everything's normal.  no problems here sir, nope

but i'm totally guessing.  have you looked at the manual?  or maybe figure out who made the playback device and check with them?   they could tell you if this is normal behavior for the thing.  because i could see how easily could working exactly as designed....

Read the Owner's Manual to find out how your particular player handles files. 

Most won't deal with a heavily branched tree. 

Keeping more than about a hundred tracks on one memory stick 

makes managing them Yourself quite a complicated matter.  

Older Chinese players have an upper limit of what memory it can see. In the early days that limit was something like the figures you are giving. Thanks to Chinese quality there are still a few being produced ready and waiting for the sellers next victim.