nth-child() doens't work

nth-child() doens't work

all. I have one question about nth-child() and nth-of-type(), I have tried
both variant, but not of them don't work. there is my code:
#commentdamcherirounded:nth-child(odd) {border-radius:4px;
-moz-border-radius:4px; -o-border-radius:4px;
-webkit-border-radius:4px; -khtml-border-radius:4px; width:680px;
display:block; margin-bottom:15px; position:relative;
margin-top:-16px; background-repeat:repeat-y; background-color:red; }
#commentdamcherirounded:nth-child(even) {border-radius:4px;
-moz-border-radius:4px; -o-border-radius:4px;
-webkit-border-radius:4px; -khtml-border-radius:4px; width:680px;
display:block; margin-bottom:15px; position:relative;
margin-top:-16px; background-repeat:repeat-y;
background-color:yellow !important; }
every background-color is red, and I want to be yellow, I tried many
variant. I don't know what to do. sorry for bad english, this isn't my
native language.