Some infants have difficulty sitting erect without some support around the chest. 

If this is the case you may find that letting your infant sit between foam blocks that provide some support on either side to provide to enable him/her sit independently and start to reach for toys. The blocks will allow the infant to start to use the arms to for support.  

In this position you can encourage the infant to lift one arm to reach for a toy.  Lifting the arm will encourage the back muscles to become more active. 

In these  pictures you see Toesies at 9 months with very poor sitting skills. At the beginning of the 20 minute play session he was  leaning back and not putting any effort into sitting erect. 

As we continued to play the catch the scarf scarf game he became more active and started to move away from the back support as he reached for the scarf..

Sometimes he leaned over too far and I needed to move him back into a more upright position;

After a while he started to sit more erect and actively use the blocks for support as he reached reached for and played with different toys.

Learning a new skill takes time

Remember that learning a new skill takes time and practice. The infant needs to figure out how ro get the head, trunk, arms and legs to work together in a coordinated fashion to achieve a goal.

Your job is to provide the incentives that will motivate your infant to keep moving. Reaching for toys is one of the best ways to an infant to be active. Always have a variety of different toys at hand to keep your infant interested and working hard. 

Be patient and allow time for improvement to happen.