Data communication via serial port (or any communication for that matter) takes time. The receiver must receive, process and reply to the data send by the sender. I suggest having a look at the examples in the Example Finder, they are well documented. Search for "Simple Serial" or just "Serial".
Your VI sends data and immediately reads however many "Bytes at port" are available. There is no delay between sending and reading:
Here I modified your implementation such that it waits up to one second for data to arrive: