Paper will contract or expand with changes in temperature and humidity. This is why it is very important to never constrain the art work by taping around the perimeter of the paper. This devalues the art work. The work should be suspended by tabs or hinges to allow the paper to move. Glass should never be put directly on top of the art work.
Moisture can condense on the glass and cause damage to the art work.
Art on paper should be separated from the glass by either an acid free mat or by the use of a spacer in the frame. If the piece has been hinged properly or tabbed properly and still won't lie flat, then it could be the artwork itself. The bottom line is that all paper will move due to atmospheric conditions. Part of owning original art work is that you are going to have slight ripples in the paper at different times of year. Please submit your answers with your questions or comments.