The usual reasons are: the price moved more than your slippage allows, you do not have enough of the chain's coin left for the network fee, the order is large and moves the price too much, or your wallet is on a different network than the coin. A new coin can also be not open for trading yet.
A failed transaction can still cost the network fee. The trade panel and your wallet show the actual error. Do not report a failed trade as abuse. Abuse reports are for identity problems.