menu – left sidebar: dissapearing items

I am using primary menu and a secondary menu in left sidebar. There is a lot of items in secondary menu – like 10-15 items, each has 0-10 child items, which opens to the right (it has just two degrees).

Unfortunately it happens again and again, that I add some items to the menu, arrange it the way I need, save it and then I see, there are items missing! I have tried a lot – but it’s still messing the items in the menu, some are missing, some are moved.

I really don’t know what to do to have the menu the way I need. ??