Accountability is tricky and it depends on the structure of the team. In a matrix environment, there are several tactics I use. Introducing yourself to the various functional managers that are providing team members is a must. Doing so, shows the functional manager you take the project seriously. Make sure to make an appointment with the functional manager and know your stuff. You are taking time from a busy person. At the end, try to achieve buyin from the functional manager that they will dedicate the right resources.
As a follow-up, have the project sponsor contact each functional manager as well. Make sure to advise the project sponsor whether the functional manager was supporting the needs or seemed to be hesitant. Remember the sponsor may be a peer or even senior to the functional manager. The sponsor may be able to turn around those hesitant functional managers.
Keep the functional manager informed!
Work with the functional manager and HR to have input to the team members evaluation. The evaluation will provide the project manager some leverage. This should be your opportunity to help someone shine!
When the project team reports directly to you then you have the opportunity to make the team shine. Remember you as the project manager set the example. Do not punish the team for the faults of a couple on the project team. Deal with them separately. Make sure that each member of the team knows the rules and processes and then make sure they are followed.
Most of all work hard and be a leader.