You aren't using to Arduino to power the servo, are you? Arduino connection to a servo should ideally be signal and ground only.
Also, Hitec Servos can be finicky. The servo library sets servo position one of two ways: by angle, as you've done or with writeMicroseconds() where the argument is typically between about 1000-2000 microseconds, 1000 corresponding to about 0 degrees, 1500 to 90 degrees, 2000 to 180 degrees.
Hitec has some digital servos where the servo only responds to a limited range, something like 1500-2000 or so. I have had Hitec ones whose range was 600-2400 uS. Maybe try using
servoA.writeMicroseconds(1500);
instead of
servoA.write(90);
to see, or use both and compare. Start with values closer to 1500 first to avoid overdriving the servo and it may take a little trial and error.
Check the servo datasheet too, but if the two lines I just supplied aren't about the same, ie midpoint, then the servo timing is one of those finicky Hitec ones.