Not necessarily, Sam. The render options can be buggy going back to at least FCP5. The workarounds include:
1. Alternate between APPLE-R (Render Selection) and OPTION-R (Render All).
2. Select what you want first (or de-select, if you had already selected), then try the two commands again.
3. Use Clip Enable/Disable (CONTROL-B) to turn off something small in a section you want to render. Turn it back on and retry the render.
4. Disable all audio render options and use only OPTION-APPLE-R (Audio Mixdown) for audio renders.
5. Use Render Manager to dump render files and re-render everything.
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