Welcome to the forums formdev1,
To start, the placeholder attribute does not have very good browser support, 62.24%
to be exact. http://caniuse.com/input-placeholder
Secondly, that is the natural functionality of the placeholder attribute, it disappears when you start entering text
rather than onfocus.
If you are looking to accomplish a placeholder that is cross-browser
<input type="text" name="fname" size="25" placeholder="Username" onfocus="this.placeholder = ''" onblur="this.placeholder = 'Username'" />
All the best,