You must make sure the ID3 tags/metadata are fully entered correctly. UPnP browsing searches/looks at file metadata. If a track is limited on metadata, then this is the reason it is not universally indexed on the UPnP Server.
If for example, the Artist and Album tags are blank, then these of course cannot be searched for. In this case, many UPnP servers offer a search by FOLDER option, which mirrors the folder layout (rather than searching more easily by Album or Artist) of the music folder on the server.
You can use many utilities like Media Monkey/Asset/Audacity to lookup or drag metadata/ID3 tags into all of your files very easily.
Additionally, some NAS Drive UPnP server do not support .WAV metadata and will default the artist and album as "unknown".
FLAC is a better alternative to get around this issue as it supports metadata.
For more information on metadata correctly, please see the FAQ here