Sometimes an order goes wrong. A package does not arrive, arrives short, or arrives unusable. The dispute process is the way back.
When to open
Open when a vendor has gone quiet past a reasonable window after a clear question, when the order is well past the advertised time with no update, or when what arrived does not match the listing. Not before.
What a moderator reads
Your account of the problem, your evidence, the full message thread, and the vendor's dispute history. The evidence you bring up front is what the decision turns on. Clear photos and a calm factual account help you.
Improving your odds
Keep the language factual, not accusatory. Answer moderator questions quickly. Do not open several disputes at once. And never finalise early in the hope a vendor will reward you by shipping, because that removes the dispute path.