As more people start to use Bitcoin, the block size reaches its limit, leading to a crowded Bitcoin network. Consequently, low-fee transactions are delayed, and sometimes even dropped (purged) from the confirmation queue (mempool).
It often occurs that the network becomes congested immediately after you have sent your transaction, especially when the price of Bitcoin is rapidly rising. In such cases, even if your initial fee was sufficient, it may no longer be adequate due to changed market conditions.
In such situations, you need to take measures to expedite your transaction. You can use a transaction accelerator, RBF (if the transaction supports it), or CPFP (if you control any of the transaction's outputs). Otherwise, you may have to wait days, weeks, or even months for the transaction to be confirmed.