Well to be honest, from what you're describing this seems to be a Wifi signal coverage problem, there's not much you can do other than add more access points to cover the shadow areas.
But I suggest that you hire a company to carry out a site survey on site, so that they can check what may be affecting your network and suggest appropriate improvements.
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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