Project (in 6.0 because 7.0 doesn't work for my projects; it fails loading avc files) is structured like this :

Main Menu selects several video tracks, as well as a couple sub menus.

The sub menus each select a sub-sub menu consisting of a still picture and a button to return back to the sub menu.
The video tracks have buttons as well, that jump to one of the sub-sub menu's with the still pictures.

Scenario is this : you play the disk, select a video track, the button on the video track appears and works normally.
From the main menu, navigate to a sub menu, then select a sub-sub menu (with a still picture) and then return back to the main menu and select the video track again.

The video track's button is gone, and never appears again. The only recover is a full-stop, eject the disc, and reload.

Anyone got any ideas?

I've tried setting everything to "auto" as well as setting everything to explicit button selections, makes no difference either way.
Something in entering those sub-sub menus causes the video track buttons to cease working.