Try holding her arms down or something, keep her too busy to pull the sheets up!

Dont force her. Sex is naturally a private thing. So she wants to cover it up.
Explain to her youd like to try sex above covers, it may not be a big thing to you but it could be for her.

Relax the environment a bit ensure all doors are closed, if she has phobias about 'walk-ins' put a lock on the door. Close the curtains if you dont.

Another way is to get crafty, catch her off guard in a way you know that she likes whilst changing the sheets for example. Just remember sex is a two way thing, if she doesnt want it, you arent going to get it.

Seems she's being shy about her body.
So, begin with body worship and erotic massage, ignoring the sheet, and move onto to cunnilingus and G-Spot caressing in fact make it impossible for her to be covered by anything other than you - make the sheet a hinderance that you kick out of the way periodically. Proceed to wild intercourse in two positions. A wild laughing romp in the bed that you both heartily enjoy!
Then when done snuggle up with her in your ams and the sheet back over top of you both.