Auto transmission trouble shifting

[cppursell]

Hi.
My 1967 Chevy C10 has a three-speed auto transmission. It seems to work perfectly fine, but on cold mornings when I take off for school, I shift into reverse, or drive, but the transmission doesn't "click" and the car won't go anywhere. I can maybe get it to move a tiny bit if I rev the gas quite a bit, but I have to either wait a while or coast for a while until the transmission finally shifts and starts driving the car. I can't remember if it also does it on warm mornings, but there hasn't been one in a while. Is this behavior normal, due to the cold (45-50 Fahrenheit), or is something wrong? BTW I have checked the fluid level and it seems fine.