On the Ethereum blockchain transactions are being processed sequentially, meaning transaction #14 will be processed before #15 and so forth. It is also not possible to process transactions with the same sequence number. it is not possible to process 14 transactions which have sequence #15, one of them will be dropped. Ethereum transactions are not being processed is that they are not getting picked up by a miner. you need to understand that miners receive the transaction and code-processing fees (gas) for each block of transactions they process, it is part of their reward. If your transaction is not being mined it means that there are enough other transactions on the network that are willing to pay a higher fee to the miner to get processed. This means if the network is very busy the transaction costs will be higher and vice versa.