Alternate explanation: Initially, more than one airline figured all they had to do was inspect things, and the plane could go back into service. Alaska Airlines did exactly that: just hours after the incident (early Saturday evening), they started inspecting their planes, gave some the thumbs-up, and off they went the next day, carrying passengers (Sunday morning). Then the FAA stepped in and said oh-no-you-don't, the inspections must follow approved procedures, etc. etc. Alaska re-grounded their MAX9s, and now they're all sitting on the ground with pieces removed for inspection access...and the planes are still grounded waiting on the FAA (who is waiting on Boeing, or vice-versa)...and still (as of Wednesday evening) there's no approved inspection procedure from FAA/Boeing for the airlines to follow.

FWIW today Alaska announced all their MAX9s would be grounded through Saturday (which I think may still be very optimistic).

Maybe in the immediate wake of the incident, COPA did what Alaska did: a quick inspection, gave a shrug, buttoned up the access panels, and started boarding passengers again. Maybe they started flying and continued flying those planes for a while. But now, they seem to be cancelling flights (though not very far in advance). Maybe the CS agents working the phones just haven't got the updated talking points yet. Beats me. But if my COPA flight was scheduled tomorrow, I would want to have a Plan B in my back pocket. Good luck with yours.