Without a lot more information it would be impossible for anyone to help solve the issue you are having.
More information would include the following:
- Shotcut version number (Help - About Shotcut)
-
Video Mode used
- To view Video Mode settings, click on Output, then Properties
- Computer specifications (CPU/Ram) (Minimum system requirements)
- Operating system (Windows/macOS/Linux)
- Source video specifications as shown in Shotcut’s Property window
- Click on a video clip, then click on Properties
- Did you use Advanced with Export settings? If so, what did you change?
- Export settings, is the Use hardware encoder option checked or unchecked?
- Filters used