Why does the sphinx not find results for the query "sofa" when there is an entity "sofas" and it finds them if you enter "sofas"?

Question

Sphinx

Hello.
There is a project on bitrix with a furniture catalog. There are sections "Sofas" "Sofas for children" "Corner sofas" and various sofas inside. The catalog is indexed by sphinx with products and sections.
And I'm looking only for sections, without goods.
1. If I drive in "sofa", then there are no results (0).
2. If I drive in "diva" I get the issue.
3. I drive in "sofas" and get the issue.
The question is why "sofa" does not work? And how to fix it?