Why buddy-codes stop working prior they expire?

I had a conversation with admin of http://hk.rc-international.info/swarm/ to find out the cause:
  • if the price for the current user is lower than price after discount, buddy-code won't work, what has sense. Usually platinum users can face this situation.
  • and the second reason, if the code has been used six times (some sources say three times). It means each code has some eligible number of purchases it can autorise. Currently we are working on determination of the exact number.