Finder question: missing expansion triangle in list view

I have a list of folders shown in list view mode in Finder. But for some weird reason the regular click to expand triangle button which is present on each folder icon has disappeared. This issue is only sticky in one of the many folders in the system.

Do you know how I can re-enable it back?

Thanks

The window may be using one of the "Arrange By" options. Disclosure triangles are not available in "arranged" views. If that's the case, you can choose View → Arrange By → None or press ^ control⌘ command0.

On Mojave the concept is called "Group" instead of "Arrange". Keyboard shortcut is the same. Menu item is found as follows: View → Use Groups.

Try to do this. Worked fine for me .

View > Arrange By > None