I always get 204 while trying to search for an artifact via the API.
example url: http://archiva.mydomain.com/restServices/archivaServices/searchService/artifact?a=test&g=com.mydomain.test&v=1.0
The groupId, artifactId and version all match up in the pom and also show up on the archiva web interface properly.
In order to download this artifact directly I use this link: http://archiva.mydomain.com/repository/internal/com/mydomain/test/test/1.0/test-1.0.war