Big Mystery

FCKeditor 2.0FC installed on (3) separate TLD domains on same virtual server (completely independent of each other). Installation directories and paths on all three are exactly the same. The same FCK install files and config were used for all three domains.
Two of the installs, everything works perfectly. On the 3rd site everything EXCEPT SpellerPages works perfectly. On the 3rd site the spell popup window opens but is empty. fck_spellerpages.html loads and loads fckblank.html in the iframe but spellChecker.js is not being called through the window.onload = function()
Any Suggestions on why this is happening????