Work down this list. Re-run after each step: click Apply Changes, or type the change into the refine bar.
Required skills. Untick required on everything except your 1–2 real must-haves. This fixes most cases.
A broad skill is required. "Front end development", "IT", "management" as required removes people who never wrote those words. Untick it, and put the concept in the brief.
Excluded skills. Check nothing important is on exclude.
Experience range. Widen the slider. "6 to 30+" is safer than "8 to 12".
Location. Widen the radius, add neighbouring cities, or remove the location and mention it in the brief as a preference. Candidates without a clean address in Bullhorn can't be matched by distance.
Bullhorn filters. A filter on a sparsely populated field removes almost everyone. Remove it and check with "has any value" first.
Two roles in one search. "Developer and project manager" halves the pool twice. Split it.
Very short job description (Bullhorn). Under about 30 characters there is nothing to extract. Add requirements to the job and reopen the tab.
Nothing helped. Click New Search, type one sentence with 3 requirements, and rebuild from there.
Quick check in the refine bar
Type "remove the location". Results appear? Location was the problem.
Type "make Java optional". Results appear? Required skills were the problem.
Still stuck
Send support the Search URL (Copy Search URL) and the name of a candidate you expected to see. That is enough for us to reproduce it.