When turning and going up a hill and letting off the gas transmission will not down shift?

I live in the mountains and when i turn to go up a road and let off the gas truck will not down shift and i have a loss of power. I've did a lot of things, tune up cleaned mass air flow sensor, my mechanic put the test equipment on my truck instrument doesn't show anything. I can't figure out if it's a transmission problem or an electrical problem, truck runs fine and has plenty of power and shifts fine thanks for the help. Ray