Yea...that's the CX blocking system for you. From my observation nothing has changed with the transition. CX "helps" you when you need it the least (pre-block for a 50% full flight or less), and is useless when you actually would want it (pre-block for a flight 80% or more full). The only tried and true way to get your seat block is still to show up at the airport or Airport Express as early as humanely possible same-day and get them to call control.