The biggest reason you'll ever lose a dispute is because you didn't submit valid evidence.  Recording your gameplay is the single most important thing when playing money matches.  It's even more important than winning -- because if you win with no evidence, you might still lose your dispute.

1. No video evidence - Record or stream your entire match, start to finish.  Make sure you're linked to Twitch, YouTube, Twitter or XboxDVR.
2. No screenshots -At minimum take screenshots, but video will always out-rule screenshots.
3. Little to no explanation of the issue - Make sure you explain the situation from start to finish so the admin understands what's going on.  Don't assume all the facts are known, make sure everything is covered.
4. Didn't submit the real score - Submitting the full, correct score is extremely important.  Submitting 1-0 will never help you if you are legit.  Admins favor scores that match the evidence on hand.  If there is a mismatch, it may hurt your chances.
5. No timestamped evidence -  Show the time and date your evidence was taken.  (PSN will show exact times and Xbox shows how long ago the video/screenshot was taken). This proves when the match was played and is extremely important when you've played someone more than once.
6. Opponents gamertag wasn't visible - Each video and screenshot must show your opponent's gamertag and/or team when possible.  This proves your evidence is from playing that particular opponent.
7. You weren't aware of the game rules - Each game has a particular set of rules.  Please make sure you've read all of the rules on your match page.