Panel buttons don't work (no replies)

The buttons on my control panel either don's work or are unusabley flaky. Pushing the O.K. button may or may not flash the next menu on but only for about a second. On the single occasion that I was able to make a selection, the encoder seemed to work but the other button 'Back' does not work at all. The 20 character by 4 line LCD HD44780 type display works fine.

This panel works with Repetier firmware in the same printer with the same RAMPS controller.

Can anybody tell me where I may look for the solution to this problem

Marlin version 1.1.9
Printer controller: RAMPS1.4/Arduino Mega2560
Printer: Cartesian
Controls: Encoder ALPS 24 detent with NO push switch
Tactile NO push switch

In file Configuration.H I have selected:-

#define REPRAP_DISCOUNT_SMART_CONTROLLER

In file pins.RAMPS.h I have modified the pins for NEWPANEL and REPRAP_DISCOUNT_SMART_CONTROLLER as below

// LCD Display input pins
	 //
	 #if ENABLED(NEWPANEL)

	   #if ENABLED(REPRAP_DISCOUNT_SMART_CONTROLLER)
	     #define BTN_BACK		31	// New line **********
	     #define BEEPER_PIN		45	// was 37 **********

	     #if ENABLED(CR10_STOCKDISPLAY)
	       #define BTN_EN1		17
	       #define BTN_EN2		23
	     #else
	       #define BTN_EN1		37	// was 31 **********
	       #define BTN_EN2		35	// was 33 **********
	     #endif

	     #define BTN_ENC		33	// was 35 **********
	   //  #define SD_DETECT_PIN	49
	   //  #define KILL_PIN		41

	     #if ENABLED(BQ_LCD_SMART_CONTROLLER)
	       #define LCD_BACKLIGHT_PIN 39
	     #endif

Thanks for any assistance,

Mike