Canon speedlites 550EXII and 550EX Set up for...

Question

Cannot get my speed lites 550EXII as a master unit and my 550EX as a slave unit to fire in syn ch in TTL mode. The 550EX wants to lag behind or fire after the 550EXII (master unit ) fires. What can I do to have them both fire together with the 550EXII being the master unit on the camera.