Print Operators

I have had a problem on several occassions during the past 3 years that has to do with the ability of a domain user to add printers. I have added domain users as a member of the print operators. Once in a blue moon a domain user cannot add printers or print (access denied). I have been successful in adding the individual as a member of the print operators. Question is why would 100 people in the domain users group be able to access printers but a couple (who are in the domain user group) bedenied. Should the domain users be members of the print operators or should the print operators be a member of the domain users or neither.