AJAX On Completed is a triggered event.
My guess is that when AJAX request with tag "MYLEVEL7" (for example) is completed and at this moment your variable LVL_LOOP is not equal 7, then this event is skipped.
How many levels do you have? You can simply make something this:
AJAX On "MYLEVEL1" Completed -> Function call LoadLevelData(AJAX.LastData, 1)
AJAX On "MYLEVEL2" Completed -> Function call LoadLevelData(AJAX.LastData, 2)
AJAX On "MYLEVEL3" Completed -> Function call LoadLevelData(AJAX.LastData, 3)
etc.