Because it has screw connections, my bet is that one of the power wire is loose. I was going to use a decoder as well but really, a DPDT switch works really well so when I upgrade my layout to DCC, I didn't even bother put a decoer in it.
Cacole could be on to something. Also, is it possible you have a loose connection anywhere along the power route? That means from the origin of the feeders to the input wires of the decoder, to the output wires, to the input to the motor. If you have soldered the feeders to the dedocer, that would be my first hard look. When the turntable ever gets running, reach under to the feeders and give a gentle vibration. If it goes dead, you know where to look. Alternatively, if it isn 't working, do the same jiggle and see if it starts up.
Or, do a meter test of all circuitry segments.
Have you measured the current draw of an Atlas turntable? It may be overloading the decoder and causing its internal circuit breakers to cut off from a current overload.
Personally, I don't know why you're even trying to run an Atlas turntable with a decoder. A simple toggle switch works just as well and is a lot cheaper and easier to operate.
I have a Atlas turntable on my layout, along with a decorder a DH123P Ho mobile decorder. I have a Zephyr basic set. Now I saw that happen on the DIY network's "Working on the railroad", they had a Atlas turntable with a motor drive, along with a decorder. Now you couldn't see the name on it but, it looked like a DH123P Ho mobile decorder.
I've been using the decorder on mine. The only problem is it works fine for a day or two, then it just stops. What the book says about keeping the decorder away from the motor, I've done that. There is no heat from the decorder, everything is wired up right because before it stops it works great. Has anyone esle used or has a atlas turntable along with a decorder to run it?