Yes this is normal. These forms are designed to have flexibility in it so that your piece can pop out easily. We simply use clamps to make sure the wood piece sits flat to the bottom of the form. Depending on the layout of your wood will depend on you clamp it down. See this video HERE for an example. If you still notice a slight twist in your piece before pouring, Clamp it to your table. Once the clamps are removed after the Resin has cured, the flex in the form will make it easy to pop our your finished piece.