Dear Mr. / Ms,
We are much concerned that your sales in recent months have fallen considerably. At first we thought this might be due to a slack market, but on looking into the matter more closely, we find that the general trend of trade during this period has been upwards.
It is possible that you are facing difficulties of which we are not aware. If so, we would like to know what we can do to help. We, therefore, look forward to receiving from you a detailed report on the situation and suggestions as to how we may help in restoring our sales to their former level.