1. Is the issue you are facing covered in the contract terms? If not, and it is material, you could renegotiate the terms.
2. How effectively are you engaging with your supplier? Have you asked them why there is sub optimal service rather than just tell them? This approach may well open up avenues that create a positive way out of the issue.
3. Is this a personality issue, or more fundamental in terms of the actual goods or services not being delivered. You may be able to mediate or even change the personalities involved in managing or delivering the service.
4. Was the issue identified in the tender when you sourced the provider, or in other potential suppliers responses? Are there any solutions to be found here?
5. If you have a multi-supplier arrangement, can you share the issue with the other provider(s) to help find a solution?