There is never a better time to demonstrate the competence and power of you as a project manager than at a well prepared Steering Board Meeting attended by the right stakeholders.
When a project manager experiences a routine, neither the project team or the management pays any attention to his requests and even the supplier resists his or her demands, it is time for action.
To reach one's ends in a smooth way without re-writing contracts or long legal negotiations, it is best to prepare a well structured presentation and call for the Steering Board members from all parties involved (here customer = PM's company and supplier). Make sure the meeting is attended by the decision makers.
Show them the agreed scope - e.g. phases with timing from the Contract. And then explain the current status. Now is the time to show them what you propose in the future - an ambitious yet feasible plan as you envision it. There should be changes to original scope highlighted and reasons properly explained. Always present an issue with proposed solution that you pre-negotiated with all parties involved. Nothing should be a novelty in your presentation to any member of the Steering Board Meeting.