The number of transactions on the Bitcoin network has steadily increased over the years. This means more blocks are filling up. And as not all transactions can be included in the blockchain straight away, backlogs form in miners’ “mempools” (a sort of “transaction queue.”)
Miners typically pick the transactions that pay the most fees and include these in their blocks first. Transactions that include lower fees are “outbid” on the so called “fee market,” and remain in miners’ mempools until a new block is found. If the transaction is outbid again, it has to wait until the next block.
This can lead to a suboptimal user experience. Transactions with too low a fee can take hours or even days to confirm, and sometimes never confirm at all.
But here is what you can do today to keep your own transaction from getting stuck...
https://bitcoinmagazine.com/articles/what-to-do-if-your-bitcoin-transaction-gets-stuck-1481042383