KitchenAid WRF989SDAM00 ice maker troubleshooting

[sonoman]

KitchenAid WRF989SDAM00 ice maker stopped working.  It appears it stopped working slowly...not suddenly.

The cold water works fine and the water supply line is not kinked

it is the slim ice-maker that fits inside the ceiling of the refrigerator and has a small door (about 1.5" by 6"), not the big one that sits below the ceiling with the big ice door. 

How do I diagnose the problem?  It feels like there is ice in the tray, but it is not sending ice cubes to the bin.  I suspect either the IR sensor or the ice-maker, but I don't want to randomly change parts out until I know what the problem is. The IR sensor is the style without the red LED.  How do I test if it is working? 

Also, there appears to be some ice in the back of the refrigerator where the rear of the ice-maker assembly attaches to the refrigerator ceiling. I have not taken the whole thing apart yet because I don't want to lose the cold filtered water while I wait for parts and time to fix this.  I was thinking the supply line could be frozen, but not sure.  . I suspect it is not a supply issue because if there are ice cubes in the tray, they would have been ejected and the tray would be empty.  Again, looking for diagnostic clues for fixing it

thanks