The first point of contact should be the person you are working with. Let them know your concerns and explain the issues you are encountering.

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In almost 99% of the cases, this should resolve the problem. If not, please escalate it to the supervisor of the person you are working with.

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If for some reason issue is still left unresolved, then this should be escalated to your client partner who is from the supplier to manage and resolve all such issues