There are mainly two reasons for a transaction to not be confirmed.
Firstly, it could be that the chosen amount of network fees that were set were too low. Generally, the higher the network fees, the earlier a transaction is performed. If only a small fee selected, the transaction might take a while longer to be added to a block in the blockchain. To avoid this occurrence from happening, we recommend that you always use our recommended pre-set fee.
Secondly, it could be that there are even more transactions with higher network fees that are performed before yours.
Find out how to check your transaction status here.