The most likely option is that the drop sold out before your transaction was confirmed.
After being admitted from the queue and initiating your transaction, we wait 45 seconds for a transaction confirmation before admitting another user from the queue (in order to prevent low gas settings or slow transactions from blocking other buyers). If the drop sells out before your transaction is confirmed, your transaction will fail.
We recommend that you avoid lowering your gas settings to ensure that your transaction completes within the 45 second window.
For more information on how our queues work, please read this article: How does the queue system work?