We had a transmission rebuilt twice and that didnt work.So we got a new one and same thing with that.Then we got one that was in someone elses truck that worked fine and the problem is its doing the same thing as the others about a year later.When driving in town at low speed it wont shift from 1st to second unless you give it gas.If it warms up seems to shift alittle better.

 

Need your truck info.

 

Chevy k1500 5.7 350 1991

 

That really sounds like a Throttle Position Sensor failure if you have to push the throttle alot to change gears.