Some songs downloaded on omnitech mp3 player won't...

Some of the Mp3 players require a particular directory structure.
If required, this directory structure is created automatically when you format the card in the player itself.
Make sure you take a backup before formatting the card as it will delete all data on the card.

After formatting, you may see few folders (unhide hidden files as well)
You need to transfer in one of these.
Also let me know how exactly transfer the songs if this does not resolve. If possible, also try giving few details.