Slow service

There are 5 tills at this branch but always only two are operating, you hardly ever find more than two tills being used at once at this store and they are quite busy which means you queue and wait so long to pay for your goods. They will see the long queue but they dont even attempt to fill another one of the open till spots. I would just like to understand if they are so many staff members, why is there always only 2 tills available for us customers to pay? Why do we end up waiting 15 to 20 minutes to pay for one item when they have staff and tills that can operate to assist more customers in less time.