When you ask for the Object Profile on a single-prim object you get three buttons at the bottom of the sidebar: Open, Pay, Buy
When you do this for a multi-prim object there is only 1 bottom button: Details
The number of prims in an object should not influence what buttons are displayed.
Change the display to have all 4 buttons showing:
Open, Pay, Buy, Details
There is enough room for them to fit by making the widths of the button boxes smaller.