One of the key objectives you have when contacting your customers' employees by telephone or email is to develop a relationship of mutual respect. This can be a really difficult objective to achieve when the person you are contacting has no respect for their own firm.
One of the core reasons why competent employees can appear to be unhelpful or unreliable is because of the culture within their own company. Not only does a negative perception of the culture affect the employee, it has a direct impact of their self-esteem.
Consequently when you contact that employee, you may at first gain an impression they are less than competent at their job. This first impression can be really difficult to dislodge from your mind. However, until you can confirm your first thoughts one way or another, you should always try to keep an open mind and look at how your contact operates over the long-term.
My suggestion is to always take a neutral and professional approach to all the people you contact. One such approach is to remember to thank them if they respond to your contacts. Always say please if you want them to do something for you. Finally, never say we did not receive (whatever you didn't receive). Say, it appears that we haven't received whatever you didn't receive.
You will be surprised if you maintain a professional approach, how often your first perception suddenly turns out to be very wrong and the person is really quite competent and helpful.
Incidentally, with Christmas and the end of the year approaching, don't forget to say thank you to your contact. A thank you at this time of year for their help, even if it was not as good as you would have liked, does wonders. Don't forget, people in Accounts Payable / Finance, rarely receive a Christmas or end of year thank you. Your thank you could be the very reward they need to help support you do better next year.
On that note, I wish you all a happy Christmas and New Year with your family and friends.