Since we are not allowed to change the algorithm, we can only play with modifying or augmenting the search query. (Note, we either change the algorithm/model or the data, here we can only change the data, in other words modifying the search query.)
Modifying the query in a way that we get results relevant to the original query: If the query is “dark pants”, results would still be relevant if it contained “black pants” as black is dark. This means we need to find results for a synonymous query too. “black pants”, “black trousers”, “dark trousers” are synonymous to “dark pants”. We don’t need to change the algorithm. So one way of increasing the recall is to also search for synonymous query by replacing words with their synonyms.
Note that you could apply the same principle as above to the result of the original query. Instead of changing the query, you get first set of results from original query, then get results which are synonymous to first set of results.