tax rate

hi,
1 I set the tax rate, I choose it when introducing the product, introduce a net price, the program calculates the gross price, I approve the "apply" and again shows the tax Product category "none" and the gross price equal to the net price.
what is it?
2 program is not translated rates:
I have two rates of 23 and 5. Once you have selected from the list, eg 23 and entered the net, the program calculates the gross price, the change rate of 23 to 5 program recalculates the gross price, which remains as the rate of 23 why?
to get your desired effect I have to delete the line "price" and add a new one.
why?