You "upgraded" from cheap rtr radios, to an equally cheap FlySky radio. I wouldn't be suprised if center on the FlySky radio was off. A servo has a range and a center point of that range. The servo horn needs to be in the right spot when the servo is physically centered. You can take the servo horns off and reposition them, but if they need to be reposition too much, you might not have enough travel.

Any radio from Airtronics, Futaba, or even Spektrum will work better than that FlySky.