In your DeaultInput file (C:\Users\username\Documents\Unreal Projects\GameTest\Config), you should have some lines that resembles something like:
-AxisConfig=(AxisKeyName=“MouseY”,AxisProperties=(DeadZone=0.f,Exponent=1.f,Sensitivity=0.07f))
+AxisMappings=(AxisName=“DoRotationY”,Key=MouseY,Scale=1.000000)
This will make sure your mouse is setup as an axis and then your DoRotationY will be picked up correctly by your SetupInputComponent. Also, is this on a character class and is the SetupInputComponent being overridden in the header?