If we mark up all materials, services and labor by 15%, then shouldn't our GPM always be 15%. What am I missing?

Thank you for reaching out. We actually specialize in mechanics liens, construction material and labor, not the sales/profit side. However, we do have this article that might provide some assistance. HERE 

If this doesn't provide any help, I would suggest reaching out to 1 of the associations (IE: Association of General Contractors) for help.