The odds are high that you have a stop marker at the end of the last item on the Timeline. If that is so, remove that stop marker. It is not needed, and it causes the problem that you describe.
There could be alternative explanations for your problem, but let us start here.
We will be watching for your results.
Depending on your reply, we may have to go into the details of your project, such as
1. Project preset
2. Properties of Source Media
3. Computer environment and resource description