The BPM Analyzer has let me down on some occasions as well. I can't help you fix it, but I can offer a comprehensive workaround.
Download the Mixmeister BPM Analyzer. It reports BPM counts for WAV, MP3, and WMA. It also tags MP3s with the information, which Mixxx can read and pull into its database. You can even export the results to a .txt file, then open it in the office application of your choice. The Mixmeister software scans your library all at once. You can turn the Mixxx analyzer off and lighten your processor load during your mixes.
It's free. There's Windows and Mac versions, and for Linux users that aren't absolute purists, it runs VERY well in Wine. Mixmeister isn't 100% foolproof, but it gets the grand majority of songs right. No BPM Analyzer is foolproof, however. You'll have to train your ear to make up for that somewhat rare bad reading.
I hope the developers give us a more reliable solution in future, as I haven't found a good free OGG Vorbis scanner with BPM tagging. Until then, this is the simplest solution I've found.http://www.mixmeister.com/bpmanalyzer/bpmanalyzer.asp