"In fact it's the Clients who order the least or are the hardest to maintain that seem to complain the most. "
Bingo. This is a type of identifiable customer - always complains, they will be slow to pay, they always want a deal, and their art is late or screwed up. They are a negative drain on the company resources, and unless they are bringing some real big or high profile jobs, they aren't worth the bother.
The only way to deal with this type is to flag them, put a DA charge on their job (DA - dicking around) and if they say anything, tell them to fuck right off and send them packing. Send them to Hazzard or Prismtech or some other printer. Spend your time on your good customers. They say ('they' being business pundits) that 80% of your work comes from 20% of your customers....so hunt for more of the 20%, and eliminate the bad ones.
I've had a few of them come back, after this treatment.....they can turn into good customers. Remember, customers are like dogs. If you don't train them, they'll shit on your floor and piss on the rose bushes.