It depends on the issue, but there are two things you can do. Note that FusionAuth typically sends the query you post to the /api/user/search endpoint with the query parameter straight through to Elasticsearch
First, ensure you are running elasticsearch and not the database search engine.
Then, for some queries you can see the generate ElasticSearch query strings by clicking on the 'advanced' option in the user search area of the admin UI. This can be helpful.
Finally, try running the query directly against elasticsearch and seeing if it works. Here are examples which will pull back all the data in your elasticsearch cluster (beware!).