Of course, nothing goes perfectly as planned. Sometimes life gets busy, routines slip, and the rules that worked in September may not work anymore by December. That doesn't mean that you failed at shared living. It simply means you need to adjust.
If something isn't working, instead of letting frustration build up, try to address it calmly and suggest an alternative. In turn, if you flatmate raises a concern, try to be an active listener. Shared living isn't just about defending your own needs, but also making space for others. Sometimes, it isn't even about changing the rules. It is about being understanding.
During busy assessment periods, my flatmates and I would sometimes help each other out without making a big deal out of it. For instance, when the other seems overwhelmed, we would put their forgotten dishes in the dishwasher or take the bins out instead of them. Keeping an open mind can be just as important as open communication.