As stated earlier, an onsite survey is the best way to verify. That being said a couple things that could be looked into, is what type of security does the SSID use? We've had environments where we have to allow WPA1 and WPA2 mixed mode for some devices. Also could be a roaming issue. What is the minimum bitrate you have set on the SSID? The default being 1 Mbps, which often makes the roaming awful between APs as the device clings to the original AP it associated to. If the devices support it, enabling 802.11r may help as well.
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