Unless the line bias valve spring is replaced with a stiffer spring... the pressure of main line will be low or slow to respond (increase) even with large boost valves as you have installed.
You see higher pressures in R , 1 and 2 gears because GM engineered the trans to boost pressure 'more' in those gears.
Did you install a .500 main boost valve? With a .282 or .296 low - rev boost valve ?
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.