Auto transmission problem

[his15153]

I have a 1969 Chevy C10 a 350 an automatic transmission. Sometimes when the truck is stopped and I accelerate, the transmission does not shift into gear. I usually have to give it a decent amount of gas for it to shift and start moving. It was happenening every once in a while. The truck has sat now for 2 months or so, and I tried driving it around the block. It happened everytime. Could this be because the fluid in the transmission is low?