Hey Judith, welcome aboard.
It could depend on what kind of "Downloads" you're trying to move to your SD card. Apps will have to stay on the tablet. Photos and documents can be moved with a file manager (ES3 is a good one - https://play.google.com/store/apps/details?id=com.estrongs.android.pop&hl=en )
I've had odd instances where it said I couldn't move a file, but could "Copy" it, so I copied it then deleted it from my device.