If you have the 4L80-E transmission and if you have a transmission problem. You have a stuck checkball at the 1-2 shifting solenoid or the shifting valve is sticking and not returning.
I will describe the operation to you.
The computer controls the first gear in this manner.
By controlling the ground switch in the computer. when the puter decides enough is enough and lets go to second then it opens that switch and denying power to the 1-2 shifting solenoid. When that happens the pressure bleeds off and the valve relaxes and into second gear you go.
Provided the ECU is reading everything correctly like engine load and rpm.