We deal with the Sales Manager but suspect the CEO is behind these problems. Replacing this supplier is impossible because they have a unique product our clients demand. How to communicate our dissatisfaction and need for urgency without fracturing or hurting the relationship? FYI they're in a developing country that differs from typical Western standards and behaviors.
This situation demands delicacy. As previously stated, choosing another suppler is impossible. Given this fact, you are, more or less, at the mercy of that supplier. You cannot threaten them to go to another supplier. Here are a few pointers when having a conversation with a supplier. First, and it may be seem obvious, but remain as polite and professional as possible. I find too often clients get too emotional when doing business, that they tend to lose their sense of professionalism. Second, when having the conversation, make sure that you show interest in THEIR business growth, as well as yours. If you show them that you are trying to continue to do a lot of business with them, they are much more likely to be receptive and listen to your feed back of their business. Finally, and this ties in with the first, but thank them for their time. Time is precious in the business world. Make sure that they know that you value and respect their time as a business professional.
I hope I was able to help you out in your situation. Feel free to reach out to me for any follow up questions.