Our supplier is a family-owned business, who's not always professional, i.e. very delayed responses to urgent questions, last minute price changes, etc. We deal with the Sales Manager but suspect the owner is behind these problems. Replacing this supplier is impossible because they have a unique product our clients demand. FYI they're in a developing country that differs from typical Western standards and behaviors.
There are 2 ways you can proceed.
1- Keeping the same supplier but improving the process.
2- Changing the supplier
The first choice and the most feasible way is to proceed with option 1 above. I assume you are far away from the supplier so it is not easy to visit the supplier anytime. Probably all the warnings, phone calls, e-mails aren't effective to change this kind of an attitude either. In this case, you can hire a local company or person to be your front face and run the project on behalf of you. Visiting your supplier regularly, giving more pressure and solving the problems and getting answers at the site directly.
And the worst case, even this may not work. If they are not willing to do the things you request, not willing to improve, then they really don't care about the business. Being in the manufacturing industry so long, there is one thing I learned that if the supplier is really not willing to do something after all the trials, efforts you make, just stop wasting time on it and move on to another one which can fit you better.
One important question here, does the design belong to them or you? If it is your design, you should get this design and move to another manufacturer. If the design belongs to your manufacturer, for sure they won't give it to you. But they are also not willing to improve your business and communications. Then do let them kill your business. Do your own design and choose your own manufacturer who is really more professional, willing to work with you.
Either way, make a change asap, don't wait for your business to be affected.
Let me know if you have any further questions, we can hop on a call. I am happy to help.