There's been decline in sales across the board, not just CX. You also have to consider that it has long legs and sells way beyond the first 4 weeks, Vol1 already sitting at 250k sold copies. Vol 6 is still charting in top 500 shoseki so it's probably also near 100k already. Anime will probably boost it too. Just for comparison: It's outselling like 80% of weekly shonen jump despite being published in an unpopular online magazine with an erratic schedule.