What exactly the reviewers mean by saying "the contribution/novelty is limited"?
Possibly related xkcd. Please don’t take offence!

I’ve seen some papers in CS which validly improve upon the state-of-the-art on particular datasets. In fact, you might find these where there is an ongoing annual challenge, or a specific dataset that is used. The numbers are good but they essentially glue two existing methods together. Sometimes there are perfectly novel contributions in the "glue": sticking these two methods together only works if you do it in a particular way, but "glue" contributions are smaller than completely novel "method" contributions and this might elicit such a response from a reviewer. Even if gluing these two methods together is novel.

Usually, in these papers, the results are great (they win the challenge/dataset) and I always wonder why the paper hasn’t been cited more. I think the answer to that is that the citations for the "method" contributions will go to the original papers and the "glue" contributions, if novel, might not transfer to other techniques or fields.