Steps:
1. Open attached project
2. Open "scene" scene
3. Select Placer prefab instance
4. Change "Placer Type" to Line in Item Placer component in Inspector
==>
Blue margins of prefabs overrides are not shown.
One of the variable is hidden completely.

Workarounds:
1. Switch to another GameObject and then back to Placer prefab instance fixes the issue above.
2. Switching to "use IMGUI Default Inspector" option in Project Settings

Please, take a look into attached movie for details