It's normal that rockbox boots to the original firmware to do USB transfers, because USB transfer not work completely right for this player on rockbox yet.
Rockbox should have no effect on USB transfer in the original firmware, so if that's not working, there could be a hardware or USB connection issue.
Are you sure it's really charging? Perhaps the original firmware is just refreshing its internal database (that also has a progress bar kind of thing)?
From experience I know that the original firmware is not responsive over USB when the database refresh is in progress. In that case, just wait a bit until the refresh is done. To shorten the database refresh, it helps to remove the uSD card from the player.