If you do find an existing thread, read the entire discussion — not just the thread title — to make sure it's relevant
If you're not sure if your issue is exactly the same, it's better to start a new thread
Syncing issues and word processor plugin issues generally require individual troubleshooting and should almost always go in new threads.
If someone makes a mistake in the forum etiquette, a *polite* correction or request to read a particular guideline would be fine.
I have started a new thread before only to be told that I shouldn't start a new thread, so it very much seems to be on a whim as to whether I should add to an old, or start a new, thread.