This may be something that already known, or maybe it is just a coincidence.

I have a very small number of files at Fotolia, and was only getting a single sale everyday or every other day. I stopped uploading to FT in May and my sales starting dropping off. In fact, I went two full weeks without a single sale during the first part of the July. I decided a few days ago to upload a handful of files and within two days I have had a half-dozen sales.

Is this just an odd coincidence, or does contributing more have an effect on my results in the search?

... does contributing more have an effect on my results in the search?

It should only have an effect on search by Date, of course, where your new images appear first. Buyers, especially those with subscriptions, may watch the newest files for images they like.

Will contributing new images change the rank of your images in the other kinds of searches, such as Best Match? I have never heard that it can. However, none of use contributors knows exactly how the searches work, so it is not impossible that some might weight your images higher in results if you are an active contributor.

I haven't uploaded in years and I do get regular sales.