Usually, the free acceleration submission may fail due to the following reasons: Fees too low: We only provide accelerator services for those with at least 0.0001BTC/KB transaction fees. Size too large: Free submission is only available for transaction with the volume ≤0.5 KB. Out of quota: 100 FREE transactions are supported per hour. Please resubmit your TXID at next new hour (e.g. 13:00 sharp) or use Paid Service if it surpasses the hourly limit. Previous transaction unconfirmed: Your transaction can’t be confirmed if any previous transactions connecting to yours are unconfirmed. Please accelerate your previous transaction first or use Paid Service instead. In addition, “Double spent” transactions are NOT available for Acceleration Service.