Many complaints fail because they contain too much frustration and too little structure. Decision-makers need to know what happened, why it matters, what evidence supports it and what outcome is being requested.
Before escalation, it is often worth building an evidence architecture: chronology, issue map, document index, correspondence trail, harm or impact summary and requested remedy.
This does not guarantee the outcome, but it makes the complaint easier to read, harder to dismiss and more useful if later reviewed by a regulator, ombudsman or professional adviser.