in my case retro is picking already previously processed element... though it should only pick an entry that wasn't processed in the past..
This is not entirely true. If a previously processed element has been changed retrospectively then it would recalculate that entry. Is this the case in this instance?
Has the element in question ever been in the element set?
I am replying from memory here as most customers are now on Enhanced Retropay as Retropay by Element is now obsolete.