JMeter stress testing the TPS can't be improved

I am using JMeter to do stress testing to my project.

I set the Constant Throughout Timer as 20/s, and last for 100s. But the actual is: the TPS most get 7/s, and never get 20/s. I don't know why is this.

I want to ask 2 questions: 1、As my understanding, the JMeter can simulate the stress so it can get TPS at 20/s, is there any config wrong of my JMeter? 2、If for other case, the Jmeter can simulate the stress only depend on the server, is it mean there is problem with my server lead to the TPS can't be improved?