The problem is not the tree structur. The field which I need is the problem. I can choose max 4 field when I choose 5 he will only take 4. And there are 3 fields which you can only choose together... is thta crazy?!
The list box probably doesn't hold field values directly. If it is a synthetic calculation, i.e. - valuelist()/valueloop() , values would not be selected. What is the expression you are using in the listbox ? Perhaps, that itself would answer your query. Or, may be there is a trigger set on that field selection.