Z axis will not respond to homing command (Mendel90)

I'm having an issue with running G28 commands on my Mendel90. Sending G28 homes the X and Y axis but Z axis does not move at all. Sending G28Z also has no response from the printer. Sending G0Z commands works to move the Z axis. Anyone have any ideas what may be wrong? Relevant portion of my configuration.h is below.

//===========================================================================
//=============================Mechanical Settings===========================
//===========================================================================

// Endstop Settings
#define ENDSTOPPULLUPS // Comment this out (using // at the start of the line) to disable the endstop pullup resistors

// The pullups are needed if you directly connect a mechanical endswitch between the signal and ground pins.
const bool X_ENDSTOPS_INVERTING = false; // set to true to invert the logic of the endstops.
const bool Y_ENDSTOPS_INVERTING = false; // set to true to invert the logic of the endstops.
const bool Z_ENDSTOPS_INVERTING = false; // set to true to invert the logic of the endstops.
//#define DISABLE_MAX_ENDSTOPS

// For Inverting Stepper Enable Pins (Active Low) use 0, Non Inverting (Active High) use 1
#define X_ENABLE_ON 0
#define Y_ENABLE_ON 0
#define Z_ENABLE_ON 0
#define E_ENABLE_ON 0 // For all extruders

// Disables axis when it's not being used.
#define DISABLE_X false
#define DISABLE_Y false
#define DISABLE_Z false
#define DISABLE_E false // For all extruders

#define INVERT_X_DIR false // for Mendel set to false, for Orca set to true
#define INVERT_Y_DIR false // for Mendel set to true, for Orca set to false
#define INVERT_Z_DIR true // for Mendel set to false, for Orca set to true
#define INVERT_E0_DIR true // for direct drive extruder v9 set to true, for geared extruder set to false
#define INVERT_E1_DIR false // for direct drive extruder v9 set to true, for geared extruder set to false
#define INVERT_E2_DIR false // for direct drive extruder v9 set to true, for geared extruder set to false

// ENDSTOP SETTINGS:
// Sets direction of endstops when homing; 1=MAX, -1=MIN
#define X_HOME_DIR -1
#define Y_HOME_DIR -1
#define Z_HOME_DIR 1

#define min_software_endstops true //If true, axis won't move to coordinates less than HOME_POS.
#define max_software_endstops true //If true, axis won't move to coordinates greater than the defined lengths below.
// Travel limits after homing
#define X_MIN_POS -100
#define X_MAX_POS 100
#define Y_MIN_POS -100
#define Y_MAX_POS 100
#define Z_MIN_POS 0
#define Z_MAX_POS (Z_HOME_POS - 0.1)

// The position of the homing switches. Use MAX_LENGTH * -0.5 if the center should be 0, 0, 0
#define X_HOME_POS (X_MIN_POS - 1)
#define Y_HOME_POS (Y_MIN_POS - 1)
#define Z_HOME_POS (203.0)

#define X_MAX_LENGTH (X_MAX_POS - X_MIN_POS)
#define Y_MAX_LENGTH (Y_MAX_POS - Y_MIN_POS)
#define Z_MAX_LENGTH (Z_MAX_POS - Z_MIN_POS)

//// MOVEMENT SETTINGS
#define NUM_AXIS 4 // The axis order in all axis related arrays is X, Y, Z, E
#define HOMING_FEEDRATE {35*60, 35*60, 2*60, 0} // set the homing speeds (mm/min)

// default settings
#define DEFAULT_AXIS_STEPS_PER_UNIT {80,80,200*16/1,E_STEPS_PER_MM} // Mendel90 with Wades and Stoffel15 bolt
#define DEFAULT_MAX_FEEDRATE {400, 400, 2, 30} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {2000,2000,150,5000} // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for skeinforge 40+, for older versions raise them a lot.

#define DEFAULT_ACCELERATION 2000 // X, Y, Z and E max acceleration in mm/s^2 for printing moves
#define DEFAULT_RETRACT_ACCELERATION 5000 // X, Y, Z and E max acceleration in mm/s^2 for r retracts

//
#define DEFAULT_XYJERK 10.0 // (mm/sec)
#define DEFAULT_ZJERK 0.0 // (mm/sec)
#define DEFAULT_EJERK 10.0 // (mm/sec)

what about Homing speeds, which you have conveniently not shown


The defaults are

// Homing speeds (mm/m)
#define HOMING_FEEDRATE_XY (50*60)
#define HOMING_FEEDRATE_Z (4*60)

Ie the last one should be 2*60

Why should it be 2*60?

thanks

Cos 4*60 is usually too fast for nema17's fitted with M5 threaded rods with a step/mm of 4000, motors just sit and squeel at you

What factors determine the step/mm?
I ask this because mine don't seem to do that.
So I'm wondering why?


thanks