Experts Please. Airbrush Salon style Tanning. Need some help?!?

My fiance has spray tanned me around 10 times. We have used clear tanner and bronzed tanner. We’ve used a Badger airbrush, a Husky touch-up paint gun and a regular car spray gun. We use a 15 gal. air compressor. The result is usually satisfactory.
My problem is that on the sites that I’ve looked at for tanning solutions say that it takes 2 to 3 ounces of solution to do one full body tan. When I tan it usually takes around 8! I know that there are alot of factors that could be determining that but I don’t know what we are doing wrong??
I’m 5’6″ 125lbs, the psi is set at 40, the solution is at factory consistency, also we’ve been using siphon feed not gravity. Is that wrong? Any suggestions welcome, thank you!

JERGENS