I don’t use that plugin but there are two ways you can do it as built-in Obsidian functionality that are pretty simple:
Step 1. Collapse the item you want to move so that everything you want to move is on one line. The carat in the gutter will be pointing right (“>”) to show it’s collapsed, and there will be a “…” at the end of the line for the same reason.
Step 2. Select the whole line, either with mouse or keyboard.
Step 3. This is where you have two options. One, you can cut (Ctrl-X or Command-X) the line and paste it where you want it, or, Two, you can drag and drop the selected line where you want it. Everything will be moved, and the group will be expanded to show everything has been moved.
I installed the Outliner plugin to test this, and it seems to be working for me. If I start with a simple outline:
I then position the cursor at the line “1”, and type CTRL-SHIFT-DOWN (the default hotkey for “Move list and sublists down”). The entire “1” section is now below “2”:
Is something different happening when you follow these steps? Or maybe I misunderstand the problem?