The main causes of unconfirmed transactions on a blockchain can vary, depending on the technical and conditional factors of a particular blockchain network. The following are common professional causes:

1. Network Congestion
Network congestion occurs when the number of transactions submitted exceeds the maximum capacity that a blockchain network can process in a given period of time.

• • Causes: A spike in transaction activity or popularity of a particular blockchain can result in network overload, where miners or validators struggle to process all transactions quickly.
  • Impact: Transactions with low fees may have to wait longer to be included in blocks as miners prioritize transactions with higher fees.

2. Low Transaction Fees
Transaction fees that are too low can make transactions slower or even delayed in the validation process.

• • Cause: The sender of the transaction chooses to set a low transaction fee to save costs, but this may mean that the transaction has to wait longer to be processed by miners.
  • Impact: Transactions with low fees may be stranded in node memory or not prioritized for inclusion in blocks by miners, especially when the network is busy.

3. Transaction Priority
Transaction priority can affect how quickly a transaction is processed by miners.

• • Cause: Some services or platforms allow users to choose transaction priority by setting higher fees. Transactions with high fees are generally processed faster as it attracts miners to include the transaction in their block.
  • Impact: Transactions with low priority or low fees may have to wait longer or even sit in the node's memory before being processed.

4. Network Latency
Network latency can affect the time it takes for a transaction to reach all nodes in the blockchain network.

• • Causes: Delays in the propagation of transactions across the network can be caused by unstable internet connections or other technical reasons.
  • Impact: Transactions may take longer to reach the nodes that validate the transaction before it is processed into a block.

5. Block Size
The block size of a blockchain can affect the total capacity of transactions that can be entered into a block.

• • Cause: Blockchains with smaller block sizes may have a limit on the number of transactions that can be entered in a given time period. This can slow down the transaction validation process if transaction requests exceed the block capacity.
  • Impact: Transactions that do not fit in the current block may have to wait to be inserted into the next block, which can increase confirmation time.