With Constraint Plan flagged and Enforce Capacity Constraints flagged, why does plan mis and move mo

I set up resources, departments, set up times, etc., collected it, ran a constrain plan with Enforce Capacity Constraints turned on. For a few months it calculates correctly where the required hours = Available hours if there is not enough capacity. But then in the middle months of the plan, the Required Hours drop to zero for no reason. And the last bucket has a hugh amount of Required Hours that should have been applied to early monthly buckets? Why?

Do I need to change a profile option? Bug?