Analyze gross profit over several months and quarters by customer. Which customers are generating it, which are rising and which are falling? What products or services have increased or fallen? List out the top 50 to 60% or more by customer and measure if these contributors are increasing or declining in monthly gross profit. Find out why they are increasing or decreasing. Is it sales mix? Is there a new competitor taking sales away? How long has it been since your raised prices? Are there new customers? This analysis will reveal most of these things when you ask your sales people why gross profit margins are up or down.