Transfer Case

I have a 2000 blazer ZR2 and when i push the 4X4 button, it try to get on but fall back on 2X4...It is not an electrical trouble, what can it be?

You need to check your vacuum lines under your hood. one runs to under the battery box to the actuator that locks and unlocks your front axle and then they travel back to the transfer case to the motor that shifts it then two comes from there I think. Check these hoses for dry rot or pinches. Check your actuator diaphragm to see if it is busted and check your motor to see if it is working.