How will the Constrained Supply Plan will honor the Alternate Resources?

The Constrained Supply Plan is not honoring the Alternate Resources. Enabled the flag for "Use Alternate Resources" in the plan options, assigned the alternate resource to the operation resource of a FG item's Work Definition. Post Collections and Plan run, the planned orders for FG item is still showing as "No" for "using alternate resource" column in the supplies and demands table. For the primary resource to get overloaded, the OH for FG item was reduced to 0 and almost 10000 quantities of forecast was loaded for that particular FG item. Any help regarding this would be appreciated. Thanks