FUEL is a transactional tax type of token, which means 2% of each transaction is taxed, half of that tax is burnt forever (making FUEL deflationary) and the other half is sent to the staking pool. Because of this, when you want to buy or sell FUEL you need to make sure you set the “slippage” to higher than 2% for the transaction to go through. You can do that by clicking on the gear icon, on either decentralized exchange, and changing the slippage to 3%.