Adobe Premiere

Okay, im trying to superimpose 2 tracks with this, but It wont work for anything. It'll work when I create the sample in premiere, where I put that yellow bar and create it, it will work. But when I export it to a video, its just black where the blue screen is, instead of what the image is I put. THe thing I dont understand is, it's there when I view the sample in the monitor, and looks fine, but in the exported video its not there. Can anyone help? THanks in advance