A restart of your Sitecore instance may cause unprocessed records to not get picked up for processing. This will result in the Path Analyzer not displaying data as they get stuck in the Generic Processing Pool. This may also lead to increased resource load as the unprocessed records never pass through to their desired state.
This issue is fixed in Sitecore 10.0.0 and there are hotfixes for versions 9.0 Update 1, 9.2.0, and 9.3.0. The fixes are available via the following: Known Issues – Path Analyzer is not showing data (sitecore.com)