wont hold boost

[Supraman]

I have a 1991 Supra Turbo. I've got all the upgrades including lexus afm, stage 3 ct-26, manual boost controller, custom exhaust,mhg,550 inj,custom fuel pressure reg,greddy blow off valve and walbro fuel pump. I noticed when I took it to the track this weekend that it would not hold boost when I would first launch it would go to 15 then drop to 11. So after the first run I turned the boost up a full turn which is equal to 4 psi. Ran down the track agian and the same thing 15 to 11. So the next run I turn the boost up again and again the same thing 15 to 11. So I checked all my hoses, vaccum lines and even check the boost controller and I cant find anything wrong. Its not smoking at all and idles good. Anybody have a clue as to what is going on? Thanks for any suggestions cause I'm stumped.