The primary cause of this is confusion around AND and OR. When to use which can be surprisingly confusing. When in doubt, trying switching your ANDs and your ORs and see if that helps.

To help you understand a little better, let's say you have two teams and each teams has one person on the team named John and several other people not named John. If you said in your query that you wanted:

people in team 1
AND people named John

You would get only the one person named John who is in team 1. That one John is the only person who fits both requirements. If, however, you had a query thusly:

people in team 1
OR people named John

Then you would get everyone who is in team 1 and both of the people named John. That is because everyone in team 1 meets the first requirement and both people named John meets the second requirement.

The most common use of OR when AND is needed is when excluding groups. If you are looking to exclude anyone who is a member of 2 groups, you would use all ANDs, e.g.:

not a member of 'Welcome Series 1' group
AND not a member of 'Welcome Series 2' group

If your query is returning zero results the best recommendation is (assuming it contains multiple clauses) to segment each clause and run a separate query. If it returns zero results and you've included it in your overall query with "and" logic then that is why the query returns zero results. Typically this will occur when attempting to query a group that has 0 members or adding a clause that would return 0 results on its own.