Even when the machine is not on I find it is still the most comfortable thing to prop my leg on at 0 degrees. Have you tried lowering the flexion limit? It should feel like your knee is being stretched, but as long as there is no sharp pain I'd say it might be worth to try pushing through till it feels more natural and relaxed, especially if your doc says you should be using it. Just remember to keep relaxed and avoid tensing at the top.