Sometimes you will find that you will need to clear your cookies. Cookles are simple text files that reside on your computer and they are used to remember stuff, like the time and date that you logged into your storeadmin. Often times, these cookies will not expire like they should and you will get an error message saying that you aren't logged in, when in fact you just did. So you need to clear them.
Here is a web site that tells you how to clear cookies in the different browsers.