I have been a devoted customer of yours for years, but I was greatly upset with the last product that I purchased from you. (insert nature of complaint here) While I have maintained a healthy business relationship with your company for years, it is now extremely strained as a result of this latest product, which I think you will agree is not up to your regular standards.
I do not like complaining about things at all, but something definitely had to be said in this situation because if this level of quality continues, I will no longer be able to continue this relationship. I can also speak for other people who have recently conducted business with you, as they have not been happy with their recent business dealings either.
The fact of the matter is that your company is beginning to slip and, therefore, something needs to be done about it. I don't know the specifics of what is going on behind the scenes there, but something definitely needs to change internally within your company if you want to keep your customers happy.
While I wish to stay loyal to your products, as I have done throughout the years, you need to understand that I expect a certain quality in return for my money and you are no longer providing this for me.
Please clean up your act soon or I will be forced to move on.