the first things to check are the led on the sensor board and the feedback value from the sensor.
When you power on the machine you should see a rede led on the sensor board flash 3 times,if not makre sure you have the board correctly plugged in and the wiring to the duet is good.
If this is good next look at the modulated value on the web interface, use a piece of white paper do you see the value change as the paper gets closer
if this is good look at the two IR transmitters and the recieved on the board, do they look bent?
RepRapPro Mendel 3 Tricolour
RepRapPro Fisher
-Carbon Arms
-Easy adjust Carriage+effector
-axis stiffness mods
HE3D -600 delta
-Duet 0.8.5
-PanelDue
-DC42 Height probe
-RobotDigg metal components
Simplyfy3D
RS Design Spark CAD