6.5Mbps Rx speed on a basebox with Mant 19dbi 120°

a few weeks ago, i bought a Mant 5GHz sectorial of 19bdi and 120° since i had a basebox in my inventory i decided to use it, the max distance from the Mant to my farthest client is 2.8Km. with no interference at all since the Mant is set up on a mountain. witch is above 100 metes. all of my clients are using ubiquiti cpe like LBE-M5-23. the problem is that some of my clients are complaining about the speed because it changes too much. when i review the wireless registration tab, i notice that most of them have 6.5 Mbpx on Rx but Tx is ok. i have change frequencies, alignment on clients cpe. i dont know what else i can do..

please some help
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i would really appreciate your help. or ideas to get better.

oscargrodri