After 100% finishing start again from the begining

I have been using bitcomet 1.27 for the last 3 months. but today i have found a problem that after just finishing 100% download a movie file it start again from the begining again. Previously I can preview the movie after completion 95% now I cannot preview the movie I wounder how this happen. The Question is why this happen? And can I backup the file or resume it at 98%? Please help me.