Each yoga session is different for me. Some days I'm more flexible than others. Some days I balance better than others. The flexibility has more to do with the time of day - stiffer in the early morning, more flexible as the day goes by and I move my body more. The balancing ability, on the other hand, has everything to do with my state of mind, how balanced I am internally, how focused I am. Some days, when I try to do tree pose I can only get my leg to ankle height, and still I wobble and have to put my toe down periodically. Other days, as if by magic, I get my leg all the way up to rest against my thigh and I stand in suspended stillness.
The more scattered or agitated I am, and the less balanced my state of mind, the more difficult the balancing poses are. The more calm my state of mind, the better those poses work. Most important, I find, is to let go of straining or willing myself to get somewhere. Instead, I pick a neutral focal point in mid-distance, maybe a nail on the wall or a light switch, and use this to keep focused on the pose instead of watching my thoughts galloping through my head. The less I strive to create a perfect tree pose and simply follow wherever my body takes me, the better. Then it becomes like a meditation in action.