Problem with Credit

Hi there,
I've a problem with Classifieds....I found that when Someone insert buy some credit and I create an AD..the system work correctly and it use the credit in depend of the week of the AD... That's work properly ...The problem is..When a User...after he has insert the ad...he try to modify it...He can't..but when he save ..the credit go down as before....
This is not normal.. Can you hel me ????