A/c Compressor Get No Command After Cool

[pro94lt]

99 gmc z71, 5.3 extended cab with 170k. A/C system is fully charged and blows ice cold in the 40's. When you first get in the vehicle as all of us do we have the a/c fan turning full blast, cab cools down and you begin turning it down-5,4,3,2 and then the compressor does not get the supplied voltage to jump the clutch in. I have replaced2 switches, one on the drier, one down low by the frame( i had acces to these at a salvage yard no cost to me). Am I missing something? i doubt it's freezing, it has a large pool of water under it after a few min of run time???? is their another pressure switch somewhere? Help it's hot here in Arkansas oh also if you turn it off and on it will come back on and it's not overcharged. Help what do i do???