Elimination does not occur in all months - not Org by Period

The application is on 11.1.2.3. Data exists in a given intersection for 10 months. For the first six months elimination does not occur as it should but in the seventh month it does. There are custom consolidation rules that post the elimination to a C3 member but otherwise the rules are standard. Ownership is not used, this not Org by Period, nothing is locked, no process management. When the consol rules are removed the eliminations behave normally. There is nothing in the consol rules that references period or year.

Frankly I'm stumped. Any idea what is going on?