It’s likely your bet has been made void for one of the following reasons:
Your bet was placed when the market should have been closed, e.g. You place a bet on Team A to score goal number 2 after that goal had already been scored.
Obvious (palpable) errors
You placed a bet at a price which was deemed an obvious error in price.
You opened a new account after self excluding and placed bets within your exclusion period. All your transactions will be reversed regardless of profit/loss.
Postponement / Abandoned Games
Please check individual sport rules, but in general postponed or abandoned matches will lead to bets being made void.