Did you try something like this using your RX/TX to see what the min/max values sent over the radio set are?
void checkThrottle() {
int throttle = pulseIn(motorInput, 20000); // see below
myESC.writeMicroseconds(throttle);
Serial.println(throttle);
}
from above: 20000 may need to be 22000 or 25000 depending on your radio set for best results. With Spektrum radios, iirc, DSM2 protocol is 20000 and DSMX is 22000