Salesperson commission calculate change

The company calculate salesperson commission rate is by salesperson. Such as Salesperson A has a commission rate 10%, then he can get 10% commission from his all invoice. Now the company wants to change the rule, the commission is calculated by items that salesperson sell out. Such as Item A has a commission rate  8% (by price), and Item B has another commission rate 10%.

Then what I do for this change:

1.Set all salesperson commission rate to 0%

2.Set all items' commission rate

3.create a new Salesperson commission report.

But after I finished all above , the commission amount isn't changed!

What's wrong?

Thanks