Line is in closed status but the reservation does't get released for a bill only order with inventor

HiThis issue is with regard to the bill only sales orders with inventory interface workflow.

On booking these types of sales orders, the line status successfully progresses to closed and the material transactions have occurred as required except the reservations getting released against that sales order.

The transaction manager is active and still facing this issue. 

This issue is intermittent. Some times there are reservations existing for an order with closed line status and some times there are no reservations.

Any help or pointers is highly appreciated.