Bornakk has acknowledged on the official forums that there's an issue with candy buckets that is affecting the Trick or Treat achievements for the Hallow's End event. Basically, looting from certain candy buckets in Northrend will prevent you from looting some buckets in the Eastern Kingdoms, Kalimdor and Outlands. "The current way to work around this is to get the candy buckets from Eastern Kingdoms, Kalimdor, and Outland before doing any in Northrend," Bornakk said.
Blizzard's currently working on a resolution for the problem and the team is asking players not to submit tickets on the issue. According to Bornakk, the devs are "actively working on a resolution for this and will provide more information when it is available."