You many need to position: relative; to get a z-index value to be used correctly.

Things with a high z-index beign hidden behind other elements with a lower z-index

If you get problems ensure that if divs etc your high z-index element is withing within have a z-index specified, it is higher than the z-index of screen elements you wish to be above.  You can give somethign a z-index of say 1000 to tell it to float above anything else, but if it is cotnainted within anything that has a lower z-index specified that will wipe outs its z-index back down to the containing z-index value!