GIMP problem, help appreciated.

I am attempting to create an animated .gif in GIMP. I have my frames all laid out, and ready to preview. However, when I preview my .gif, all the collective frames show up, so my animation has a sort of trail. The last frame of the animation also shows the first frame still showing. I want there to be no trail, so only the current frame is being displayed at one time. All of the frames are masked with transparency. Any help will be appreciated. Thank You.