The normal reasons for the shunt not working are wrong charged voltage setting, wrong wiring or doing lots of charge / discharge cycles without getting to 100% occasionally. We can help but need information.
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Wrong charged voltage. What absorption voltage do you use. What current is flowing when charging switches to float. See Charged voltage setting.
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Wrong wiring. You said you had solar, if the solar is DC where is it connected, the cables from the MPPT go to the Lynx distributor on the right. See Shunt wiring
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You say you get to 64.0V, how often and does the Lynx shunt reset to 100% when this happens.