Personalization using OM_PRE_BOOK_EVENT does not process preexisting order lines

Have created a Form personalization using the OM_PRE_BOOK_EVENT. Am observing that this event based validation does not fire for pre existing lines on the Order though they are in entered status. If for similar data a new line is entered on same order the personalization works, works on the copied orders too. Has anybody experienced a issue like this. Thanks for reading this.