Why does the negation as a failure work with \+ is_list but not with \+ is_flat?

Question

flattenList(A,B):- flattenList(A,[],B).
flattenList([],B,B).
flattenList([H|T],L,B):- flattenList(H,L1,B), flattenList(T,L,L1).

% negation as a failure "\\+/1&