As your grandmother used to say, “You can catch more flies with honey than with vinegar.” This is the attitude you should use when establishing rapport with your trading partners. It also works well when dealing with most situations.
This doesn’t mean that you have to bend over backwards and acquiesce, but if you are reasonable and have your facts together before speaking with them, it will go a long way. Most of your trading partners are just like you; they want to get the job done with the least amount of problems and want to work with you in getting issues resolved.
If you go into the situation yelling and demanding, you have already lost the battle long term. It might get your current issue resolved, but in the future they might be less likely to work with you as fast for resolutions. Having the issue well researched on your end before contacting your trading partner puts you well ahead of the game. You can talk with them intelligently regarding the perceived issue, and they have more information to help research the problem on their end.
However, sometimes you reach a point where you have to escalate the situation (i.e. the urgency of the situation not being understood). Knowing when to do this is just as important as knowing when NOT to. If you have established that good rapport they are more inclined to push your problem resolution to the top of the list.
Remember though, “poor planning on your part does not constitute an emergency on theirs.”