My th400 won't shift out of 1st gear when it warms up, when you first fire the car up it will shift but when it starts to build temp it won't go into 2nd at all. Any ideas?

 

I would look for a "wrong gasket " under the detent solenoid or wrong bolts . Depending on which solenoid you have , one has a rubber seat built in and does not use a gasket . The other has a flat metal seat and takes the gasket that comes in the rebuild kit .
The early solenoid was an aluminum body with a tin cover , it took bolts that were a slight bit longer than the later solenoid that was a steel body . If you have the early bolts and a late solenoid the bolts will bottom out and the solenoid will be loose causing a governor circuit leak .