Do you have the work area bar covering only a small amount of the project? If so the rendering will only do that small area.
PRE7 help topic (PRE8 is the same).
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Please tell us more about your Assets - your source video Clips. The full specs. would be useful, along with the process that you used to get these onto your computer and then to Import them into PrE. If needed, G-Spot can give you all the file info.
Next, please tell us about the Project Preset chosen at the time you did New Project. Does this match your Assets 100%?
Last, how much defragmented free space do you have on your HDD? How is your HDD formatted, FAT-32, or NTSF? Render files can be pretty large.