  Let $k\ge 2$ be an integer and let $A$ be a set of nonnegative integers. The
representation function $R_{A,k}(n)$ for the set $A$ is the number of
representations of a nonnegative integer $n$ as the sum of $k$ terms from $A$.
Let $A(n)$ denote the counting function of $A$.Bell and Shallit recently gave a
counterexample for a conjecture of Dombi and proved that if
$A(n)=o(n^{\frac{k-2}{k}-\epsilon})$ for some $\epsilon>0$, then
$R_{\mathbb{N}\setminus A,k}(n)$ is eventually strictly increasing. In this
paper, we improve this result to $A(n)=O(n^{\frac{k-2}{k-1}})$. We also give an
example to show that this bound is best possible.
