Which one is the correct expression?

Sentence number 2 is correct. There is nothing wrong in sentence 1, except that the contracted form "didn't" should be used instead of "did not."

Or better, you can rephrase the sentence as "Why hasn't this product performed well?" or "Why didn't this product perform well?"

thanks! that answered my question.

1) If you wish to post a question in the future, PLEASE consider using the common coutesy. A " please" is welcome!

2) None of the above are idiomatic, nor are they nature sounding.

3) You may consider rephrasing "Why didn't this product perform as expected / advertised.

No article.

Pengfei: You may say (l) Why did this product NOT perform well? or (2) Why didn't this product perform well? In other words, if you wish to say, "Why did not...?" you need to use the contraction. (Hopefully, someone will explain why.) By the way, in conversation, it would be helpful to use No. 2.

How should I use "please" in my post?

Did you mean "Which one is the correct expression, Please?"

Thanks a lot for pointing out neither of these sentences are nature sounding.

AnonymousPengfei: You may say (l) Why did this product NOT perform well? or (2) Why didn't this product perform well? In other words, if you wish to say, "Why did not...?" you need to use the contraction. (Hopefully, someone will explain why.) By the way, in conversation, it would be helpful to use No. 2.

Thanks for your explanation.

PengfeiHow should I use "please" in my post?

I would say do it the same way you would normally ask people to help you....It will promote faster response.[Y]

Or any polite request like:

Can you take a look please ?

I need some help please