Not sure if this is the cause, but
int x_ax = analogRead(0);
if(x_ax <= 504) {lcd.print("R");}
if(x_ax == 507) {lcd.print("N");}
if(x_ax >= 510) {lcd.print("F");}
Your code above won't print anything if analogRead(0) returns one of these values: 505, 506, 508, 509.
int x_ax = analogRead(0);
if (x_ax <= 504) {lcd.print("R");}
else if (x_ax >= 510) {lcd.print("F");}
else { lcd.print("N");}
This prints "N" for any value in the neutral dead-band between your "R" reverse and "F" forward thresholds.