what do you have the control throw set to in your CORE? i had some servos installed in a plane what were working and the throw was set to something like 120-worked fine. then i did some adjusting and i think i set them to 128 or 127, and then they could be over powered by hand. i set them below 125 (this is fuzzy memory and after 2 beers-sorry, its been a long day of work) and then they worked fine. it was having the throw greater than a certain value on a mercury using JR 8955 servos. no problem with the Mercury-or the CORE or the JR's.
not sure this is helpful but work a check. set them to 100 and see what happens.