Have you measured between those 2 access points you give as an example? For an example an Iphone will start looking to roam at -70 but the neighboring AP needs to be at least 10 dB higher at that point before it does so. Perhaps you hare not having enough overlap. Also if you are using MA-ANT-20 make sure the AP's are not mounted over 8 meters above ground. Especially in the 5 GHz there is a large null zone at 45 degrees both above and below the antenna.
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