A novel application of algebraic logic to first-order logic is given. The last author has already proved in [Stud. Log. 72, No. 2, 285–309 (2002; Zbl 1019.03047)] that a version of the omitting types theorem fails, in a certain sense, in finite variable fragments of first-order logic; also, complete representations were discussed there. In the paper under review, a new construction is presented for representable but not completely representable atomic relation and cylindric algebras of dimension \(n\) with \(n > 2\). In particular, the constructed algebras are one-generated. This construction is used to prove a stronger version of the mentioned negative result, which goes back to a conjecture in [loc. cit.]. It is shown that one binary relation in the language suffices, and that there is no upper bound on the number of extra variables needed in the so-called witness formula. The proof of the new theorem uses methods of algebraic logic; the theorem is actually given also an algebraic form that is even stronger than the logical one.