Hi,
assuming I've tested everything else I can imagine
(details here: https://tvwbb.com/showthread.php?75023-Heatermeter-amp-Adapt-a-Damper-All-ok-but-Servo-is-not-working
May be the Rasp PI 3 B+ the cause of not working servo or randomly work for few seconds only?
I tried 0,5A, 1A, 2A power...