Transmission recently rebuilt, after one month, won't down shift to first gear. on 1997 Chevrolet 1500 Pickup

This condition starts after truck has been running around ten minutes. The local diagnostic machine said change the shift cylanoid B and we did that and it still doesn't work. Same problem exists. Any suggestions on what it could be?

Asked by for the 1997 Chevrolet 1500 Pickup