hi peacesong;
this is caused by page (or even request)-specific JS being inserted by a plugin, which results in the aggregated JS being different each time so the cached version cannot be used. you can either try to identify the culprit JS and exclude it from optimization (see FAQ) or you can tell AO not to aggregate inline JS at all using the API (using the autoptimize_js_include_inline filter to be specific, see example code in autoptimize_helper.php_example).
hope this helps,
frank