I wouldn't worry about it too much. It's fairly common, I've heard, for ball python's to go irregularly and not on a schedule. I've had mine since September 2013 and I've only found solid poop about 3 times, but he passes urates weekly.

As long as urates are passing and you can see/find them, probably nothing to stress over. Some balls poop like clockwork, others once a season, and so on. Will also depend on their eating habits.
But, as RobNJ pointed out, soaking usually helps things along if you are super stressed.

Best of luck!