how can I fix mt Lennox furnance?

My electric Lennox furnance discharges 20 + degrees warmer at one end of my house than the other end.It's a simple 1700 SF ranch.One trunk run is 35 feet long and thats the warmer end, the other trunk run is only about 12 feet and it discharges 20+ cooler than the other end.It can't be loosing 20 degrees in 12 foot of duct work.These degrees are from the end of the main trunk runs.The installer can"t help.The air velosity is the same at both ends only the temp is changed.