How long is the duct run? How is the duct designed to promote airflow? Signs of duct leakage? Too many penetrations? How many CFM's coming out of a vent? Why? Is return air insufficient?
Too many unsealed penetrations in the attic floor? Light fixtures sealed? Recessed lights? Plumbing/wiring sealed? Where is the dust and unwanted airflow coming from?
Use thermal imaging gun. Are there hot spots in ceiling? Are the exterior walls insulated properly? Is the existing insulation low or inconsistent? Too many recessed lights? Areas of house differ in temperature?
Are the plenums sized properly? How is the condition? Does it leak? Are ducts located properly on the plenum? Too many ducts attached to plenum? Too many Y's?
Are the soffits clogged? Does the attic have functioning baffles? Is the attic hatch sealed and insulated? Is the stairbox sealed in insulated? Any bedrooms share an attic wall? Complaints about dust?
Is the attic vented properly? Is homeowner using Incandescent bulbs? What about pool pumps? Is it Energy Star rated? Size of household? Behavior, a lot of cooking? How tight is the house (TEST IT)?