Stay on memory pool when I sending multiple transaction from bitcoin wallet
Bitcoin Core has locks in place that will stop multiple calls to build a transaction to be executed in parallel. Instead, they will be executed sequentially, as the lock gets relinquished by the previous call. So, the other thing that comes to mind is that there are limits to the ancestry set of transactions that may be submitted to the mempool at the same time.

Your own node's mempool will not accept more than 25 transactions or more than 101 kB in total transaction size that are chained/interdependent in some fashion. Your wallet will attempt to rebroadcast transactions to your own mempool every thirty minutes or so, so even when some of the ancestors of your transactions get confirmed, it may take a while for the others to be relayed to the network.

What you can do: If you're sending a very large number of transactions, e.g. more than 25 per block, you should split up your balance across a greater number of UTXOs, so that you can create a larger count of independent transactions.