Sometimes when you are constraining a part/assembly in Autodesk Inventor, it does not work as you expect.
I often use the Ground command for parts of assemblies where I want to attach a corresponding part/assembly.
To make multi-grounded parts/assemblies, I use the right mouse button to select needed parts/assemblies. To add or remove parts/assemblies, use the Ctrl key.
The left mouse button opens a menu. Select Ground, which turns the Ground command on or off, accordingly. Now you should see your parts attached nicely.