The most likely reason for this is Advanced Styling with Glass objects. This is a setting at document level that you can change in order to remove the gradient (glass) effect. If you go to Settings>Document Properties, on the General tab you will see options for styling. If the Styling Mode is set to 'Advanced' you will need to change the 'Sheet Object Style' property to plain in order to remove the gradient (glass) look.
I hope this helps