In most cases it is because the AP did not obtain an IP or if it did, it is unable to communicate with the Meraki Cloud.
Have you checked if the AP received an IP or any firewall logs?
Have you had to do a factory reset of the AP?
Otherwise, I suggest you open a support case.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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